There appears to be a regression in this version of Minecraft on the Switch. Before this version of the game was released, all of the below worked fine. This is not a subtle bug and should not be hard to reproduce at all.
We have tested this on multiple save game files created over years. All Nintendo Switches are on the latest available Minecraft version for that platform as wells the same OS version.
When a host Switch device autosaves a 50+ MB Minecraft world, even as few as one or two other players playing in local or online switch multiplayer, one or both non-local players will find themselves disconnecting very reliably. This happens with a variety of types of save game files.
We've played literally hundreds of other games with this networking configuration, and have played Minecraft for dozens of hours via this networking configuration over many years". This does appear to be a regression in the game and not a network troubleshooting issue.
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Context: After providing details, I will speculate a bit upon what it appears might be fruitful avenues for investigation of the issue, but this is just an experienced software engineer who's worked on networked embedded networking device issues giving a total guess but definitely not a game programmer nor one informed about the details of Minecraft networking. I know a good deal about intentionally testing and stimulating network failures and have tried to characterize the bug though, but it's basically a 90% chance of happening any time the main console autosaves, so there was limited work to do on that front 😛.
I will try to call out what appear to be the localization string constant variable names that you use for the networking issues to hopefully assist in finding them. (I see these names published on international sites when new versions come out).
Reproducing the bug:
We've done a number of local saves to check different survival worlds: They all show the bug. We have found a downloadable adventure save file we'd played together awhile back that now exhibits the failure/bug (The Legendary Phoenix). That makes a 80MB save file by the time you're done. We have not tested creative worlds.
Additionally, looking at our "completed" save game file done under previous versions of Minecraft for the marketplace scenario "Minecraft: The Mountain" we get the same kind of error. It's a marketplace adventure about the Minecraft branded book of the same name from Max Brooks. We did not start either of these adventures again with a new world, but I'm fairly certain The Mountain one is very available on the store.
I've not attached the switch data files: Those are exceedingly hard to extract correctly.
That said, the seed of our main test world is -6715184763469579863.
Our spawn (no bonus chest) was about at -545, 64, 85
The forest mansion door fairly near to -3428, 103, 280 would reliably cause a disconnection before we had defeated a vindicator in that Forest Mansion there. Now it does not do so. This might be an unrelated issue, but might not be?
Here is a easy view of the seed: https://www.chunkbase.com/apps/seed-map#-6715184763469579863
The seed of the legendary phoneix scenario save game we are using (not that it matters?) is: -189012883
The seed of the mountain scenario save game we are using (not that it matters?) is: 1572407133
To reproduce with that Mountain adventure:
Download it off the marketplace on a Nintendo switch console (Switch A)
Create a world of it
Play with it on Switch A in multiplayer so other switch friends can join
Switch B and Switch C connect in the "Online" tab to join the adventure (or really just one of them even)
Play for awhile, making sure the file gets up in the 50MB+ range (ours is in the 80MB+ range) when saving.
Await an autosave on the main console (Switch A) while Switch B and C are being played as well.
Lag (blocks not updating, doors and gates not reliably reflecting true state, mobs not accepting damage then fast-forwarding) should be experienced where player actions occur on all three consoles, but especially B and C while the autosave icon is visible on Switch A. There may be some exacerbation about moving items in chests during this or not, it's hard to say.
Consoles B and or C will likely lag out with one of the below errors.
There are multiple different connection failures possible. From the perspective of the client consoles (Switch B and C):
disconnectionScreen.unexpectedPacket approximately 20% of the time.
"Disconnected due to an unexpected type of packet received which cannot be processed. This could be due to a version mismatch or corruption at the networking level"
and
disconnect.disconnected aka approximately 75% of the time.
"Disconnected from Server"
Very occasionally, you get a the following two error messages (together on one screen) when the saving happens during an attempt to join the Switch A player on to play along.
Sometimes, you get these two error screens when in interferes with loading in while joining the game:
disconnectionScreen.timeout : "Connection timed out."
disconnectionScreen.cantConnect : "Unable to connect to world."
Sometimes, but far more rarely , we have seen "Detected lost connection" (which I do not see in online lists of your localization string constants).
[These are not the Nintendo switch "connection lost" black screens with the OK button, these are internal Minecraft screens. ]
From the perspective of the main player:
Switch A experiences partial joins sometimes as "<playername> has left the game"
Switch A experiences already fully joined players being lagged out as "<playername> has left the game"
Details/What we have tried:
This happens when in the same household reliably, and happens whether using wifi or local connection mode. Various other games play together over this wifi just fine in both modes, I am happy to verify with a specific title (But Minecraft dungeons is one, for instance, I bet you may have familiarity with).
We've definitely tried turning off all skins, and alternatively either waiting for the marketplace connection to fully establish on the main screen before connecting, or rushing right in. Neither help.
As with this version of Minecraft, you do not allow adjusting of the autosave rates, it's impossible to play in multiplayer once a save game file gets into this range.
We've done a number of local saves to check different survival worlds, and have found a downloadable adventure or two which exhibit this same behavior. We have not tested creative worlds.
All three of us own a copy of the game and we're careful to use the correct users for each console to download it.
This is the bedrock version on the Nintendo Switch. We have done a number of networking and other troubleshooting steps and I'm fairly confident this is a software issue, not a save game or local corrupted file issue (due to completely reinstalling on all devices). This failure occurs when doing local networking or online networking.
We are all connected with high signal strength to the 5ghz wifi signal and have gigabit fiber as an ISP level networking connection. We do not have non-standard MTU configurations or anything like that.
We have even tried assigning static IPs to the 3 switch consoles and trying with non-static IPs. The same save bug persists.
We have tried ensuring all three accounts are signed out of Microsoft accounts as well as ensuring Nintendo specific skins are disabled, as well as all texture packs, etc. None of these seem to effect the issue at all.
We assure you we are not doing something like going to the Switch home screen or any other number of perfectly understandable things for the switch to lag out. This is core play, or even trying to "stop doing anything it's saving".
We have tried, on a copy of the world, to enable the experimental next version features. This does not appear to change the behavior in any way with respect to this bug.
We have tried this on the SD card and on the main switch both. We've redownloaded all three switches install files. We've ensured there is a ton of spare space on both.
What's possibly going wrong?
Likely this is useless speculation to you but this does look like a networking coordination failure (aka timeouts are too low for the client) or a multi-level failure (where laggy responses or dropped packets are triggering higher level "oh no our game state is corrupt" logic above the networking level instead of asking for more retries and chilling out). Just offering it because this feels like what's going on possibly due to how slowly the game lags out (after a few seconds of saving), and due to the mix of different error messages.
1. You could have "overly strict" checking recently implemented in the "is this packet/message okay" in the client side connection, not the server side connection. I would strongly consider looking into any recently added/enabled "hack prevention" or "corruption prevention" code to see if something is now overly strict in a way which causes this extremely frequent bug.
2. You could have a responsiveness issues with respect to networking on the server side of this connection between switches/through Nintendo online when saving is going on. This likely means the clients are overly strict with determining a connection has failed with respect to how well you can deliver performance given a Switch console acting as a server. The fix for this would likely be increasing timeouts as the world size gets bigger, or just increasing them on low performance platforms in general. If nothing else, adding this an an option would very likely fix this issue right now and allow a more leisurely long term "how do we keep the games synced enough while the primary console saves".
3. Lastly, you truly could be sending out corrupt packets out of the server switch or part of a entity which is cancelled as save logic is exercised. For a complete hypothetical: it's fully conceivable if say a pig is killed, some complex behavior of the pig could get partially transmitted but the rest of the behavior would be "cancelled" from being transmitted because it's now dead. This could be correctly identified as "not all of what should have come as part of the behavior of the pig", but misidentified as "corrupt" when really it's just "ya, saving object often get partially transmitted because we just can't keep networking performance up while saving because the switch is slow at that". Or really, it could be invalid to not send all that pig state, and the server is messing up there.
Thanks for looking at this report. Hopefully it's helpful, and glad to test further specific behaviors.
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Back when the Legendary Artic Phoenix was given out for free (2-3 years ago iirc) my kids had difficulty getting it to load at all on the Switch. I think it may be the high-resolution textures that trigger the problem.
Is your main test world where you reproduce this a purely vanilla world, or are you using any resource packs?

First of all: My main motivation for providing the bug report at all. Bold is just emphasis, as this is a long post, not yelling, btw 😃:
I feel recent {}software changes, likely made for real reasons, have likely significantly degraded the for-free multiplayer experience which Minecraft has had up until very very recently. It is incredibly unlikely this was intentional, but as there are servers you sell to run the game, and you have new paid Minecraft IPs (Minecraft Legends), etc, the point should be made that it's "not a good look" for people looking to cast that aspersion (which I am definitely not).
I personally don't think these are at all linked. I think the internet, generally speaking, has a pretty dodgy grasp of correlation vs causation. Summer vacation and it's attended travel just started in many parts of the US and lots of families are going to be experiencing this bug (as realms and wifi are going to be less certain, and more local play will occur). I assume, if this bug is as widespread as I suspect it likely is, fixing this may become surprisingly high priority if that happens and wish to equip the team to do so if they choose to do so. **
As to why I pointed at the scenarios:
When we played Arctic Phoenix after getting it for free a couple years ago originally, it had basically no real problems loading for us on a 2017 switch (plugged in and shown on a TV) and 2 clients (2020 normal switch in handheld mode and a 2020 switch lite also in handheld mode). We are fairly religious about moving games to the system memory vs (the fairly high speed) SD card if there are performance issues, so that may have been the performance difference for us?
Apologies if I was unclear about why I was mentioning the scenarios: I'm not specifically charging the core Minecraft team with responsibility to make sure all these scenarios work in all situations. I felt pointing out scenarios available on the Minecraft store which quickly demonstrated this bug when 1 switch host and 2 switch clients were being used (or even 1 and 1 often), would be an easier way to reproduce than just giving a Minecraft world which is not a predictable result to get to. 1 server 1 client seems basic "gotta have that working to advertise the game has local multiplayer" level of functionality.
I'm reporting failures in very typical vanilla worlds which have very reasonable sizes recently becoming unplayably bad in local multiplayer. I bring up these store scenarios as clear exemplars of things which stimulate essentially the same bug for us. Kids playing in vanilla survival would easily get to this complexity of a world in a couple days of plays.
We have experienced this on multiple vanilla worlds as well. The main test world is a vanilla world and we actually only checked the Legendary Phoenix and Minecraft: The Mountain to try to find an easy reproduction to show what would cause this bug as it really did feel like a general performance degradation of the core stack, and not a "world based problem".
As in, I thought the test team could restart the Mountain scenario repeatedly to see what the threshold is for failure as it quickly gets to these sizes, but the vanilla world we most recently worked on, for instances, would take many hours of replay to get it to a threshold place which definitely causes the behavior and it might not if you don't do something specific with animals or the nether or a village etc. It only takes about 15-25 minutes of play for even a 7 year old, for instance, to get Minecraft: The Mountain far enough along to cause this issue. We saw the issue by the time we got to the "Collect these 6 items quest". I assume a professional tester would take about 7 minutes to repeatedly get to that point.
I say this because the fault (saving becoming so overly prioritized it boots clients after causing lots of lag), is very likely (from a software perspective) not simply "exactly 50MB" and you'll definitely see it. Serialization of a Minecraft world to a file possibly takes longer based on specific complexity and specific happenings in world, and I knew both of these scenarios had a lot going on there and would be likely to demonstrate it. It may not even be serialization, but the snapshotting or something like that. I just wanted a state easy to get to and to get to similar states repeatedly which do not exhibit the error as well. For instance, at the beginning of that scenario, for instance, all 3 people can play and the saves do not lag us out, take an undue amount of time, nor anything like that. The scenario itself (if you follow it's directions) also quickly tells you what to do to reproduce the bugs (as you just play with 3 switches enabled and you'll see it happen after not that long).
In our recent vanilla world, there are a few animal pens, some growing crops, and a things like that near our home base. Nothing YouTube worthy, but nothing "this should clearly break the game". We probably haven't built a mine cart yet and the most red stone we have is 5 observers on a tiny automated sugar cane greenhouse which is tiny (like 5-6 stalks wide, one line) and textbook. But the fact there are X sheep or Y cows could be the issue in that world, rather than the exact number, and we'd never know. (Note: being far away/in the nether while the animals and crops exist does nothing to help the bug). We moved some work tables around in a village fairly far away, and discovered a Forest Mansion and built a long cobblestone path through the nether. We built a couple mines (not automated). We have a box which automatically composts stuff you put in a hopper, and a similar box which automatically cooks food you put in a different hopper. Essentially very basic, not very demanding (from what I understand) of a world.
Many of our other former worlds have much more in the way of minecarts and things like that I feel would likely exacerbate the issue/make people go "it's clearly the whatever which is too much", but I'm willing to investigate if that would be helpful for some reason. We built this world "light on the machine" per se, from what we understood.
If uploading our particular file of this particular vanilla world would be that helpful, I'm happy to do it; I just need to link my daughter's account, upload it to a realm, then download it on a desktop computer. Hell, I'm happy to connect up a debug mode client or something locally on a Mac or windows computer and export logs. I am a professional programer (mostly iPhone) and am willing to climb down the "enable all the log files" rabbit hole if it's of use next to a system reliably exhibiting the fault.
I'd honestly expected this bug report to be "oh, that's a better reproduction for [Known bug so and so] than the one we have" and just be hot linked to a dupe in the JIRA. I really don't expect this to be a unique bug at all: I think this is a performance regression, probably deeper than was intended, and the team is likely partially aware of the chance of this happening, just not how quickly it does. Just tried to bring value to show "Look, all this stuff that used to work now doesn't, things really are getting worse, and likely past the commercially acceptable point all things considered".
For instance: Despite repeated 10x - 20x times we logged into the game from the switch lite above on 6/4/23, the Nintendo Error log only shows 2 error 2306-0332 that day (that's the black screen switch error you get when the network disconnects). Those two errors are when hard shutting down the Minecraft software (home button then X), not the game. When in the past we've ever seen a game repeatedly have network issues, we've seen those happen far more in these kinds of error logs.
I'm happy to provide account IDs and switch IDs the appropriate way and upload it if there are error reporting analytics to correlate to. If you have a document about "How you don't get pwned when helping us solve bugs and providing PII!" document to make sure I do that the "appropriately secret way", let me know. Re bog standard JIRA, "Viewable by All Users" toggle will not let me select any group other than "users" when changing who can see comments. so I bet "just drop that info in a comment" is not the right way to provide that data with this particular site. Happy to become more well informed as well on "how Minecraft works" to try to repro better if needed.

One relevant point I'm not sure I said: The host for this game does not have a Microsoft account (yet). The secondary and tertiary Switches were played by players who do have one, but the errors occur whether or not those players were logged into the MS account or not.

This issue seems pretty close to what I'm experiencing so I'll provide some additional details that I noticed.
I play on the switch with some friends and family. My world was made in 1.17 and my sister made hers in 1.19. We'd frequently play together without problems. Ever since the recent 1.20 update, when we join each other, we can only play for a couple of minutes before the non-host player loses connection. (Also when joining the world, the non-host player sometimes has a wiped inventory, as if they're joining for the first time.)
Single-player in our older worlds works just fine.
Tonight we made a brand new world and everything worked perfectly fine and we had no connection issues.
The bug might be size related, like OP said, but it could also be related to something in the new update breaking or lagging older worlds. (Not sure what that might include though especially since it's a multiplayer only issue.)

I've been having issues like this for a few days now, i often play online multiplayer and our group has finally settled on a world to put some hard graft into. we initially believed it to be down to the fact we were spread quite far across the map, i for instance being somewhere around x:1500, z:1500 and the home base being at ×:2, z:-400. the game was having a hard time loading chunks and my friends would die due to a mob they couldnt see or a fall and then unable to rejoin. it has however gotten worse in the past day or two regardless of map location, i (being the host) get a notification: named player has drowned for instance, while that player is in fact live and on dry land. in this case the other player has the decreasing air bubbles and has to watch themselves die only then to be unable to join without multiple game/console resets. there have been occasions where players are unable to interact with chests/beds/blocks before being kicked and its gotten to a point where at least one other player can anticipate a kick due to lag. there's never more than 4 of us max and this occurs even with just 2 of us, though it is less laggy with less players. its literally making the game unplayable when every hour we spend a good 20 minutes trying to get everyone back in the game, loading and reloading.
we've also on multiple occasions now, myself included, lost inventory on respawn. not too much bother most times, but some valuable items have been lost that will take hours more gameplay to replicate. one player has turned to joining the game from their xbox as the switch would not log in nor accept invites but its just as bad there.

+1, but on iOS , iPhones
Me (second player) and my sister (world host) have a minecraft world. I join through our accounts, not LAN, and I get disconnected constantly. No constant trigger or time between disconnections. Our phones both work completely fine and it is not a WiFi/internet issue. I tried deleting and reinstalling my app, successfully logged back in through xbox live and joined her again and it only made the disconnections more frequent. We have no external resource packs or mods applied.

Something about the lag I'm experienceing has changed from .60 to .62. Everytime I look in the direction of the host player on Xbox, massive lag happens. However, the same seems to happen when the host comes near me. It seems to be worse when the player doesn't move for a little while.
Edit: I forgot to mention the change itself: .60 things improved but .62 brought it back.
Is Minecraft installed and saving its data on your device's internal storage or an external micro SD card?

Internal storage

*strong text*Internal and SD card storage both show this issue. Multiple different switches as host show this issue. Using mesh wifi (eero) or direct wifi (google fiber, dlink, att) show this issue. Using resource packs or no shows this issue. Using no packs, simplest fast wifi and “not janky machines” eventually still shows this issue.
I only mentioned scenarios and such as it was fast to reproduce. There was not a workaround, very basic small farms or any standard exploration would eventually show the same issue.
You can repeatedly stimulate this issue with small farms (a few foods grown, then a pen for animals which you can walk around and toss lots of food), then send one of the other two players out of chunk, perhaps through a portal to another area. The whole goal is getting the sync effort high. The configuration of three players on local network sync on three switches with a ~50 mb save file alone will repro the error when done naturalistically. I do not know how big a “trivially” generated large save file (like overly exploring with a boat or something) would have to be to generate the error but basic minecraft “build a base” stuff fails after a couple of days of play with this limitation.
We have tried a few nintendo switch versions to see if the error is corrected. We can do so again periodically. Right now, minecraft was MIA in our gaming rotation though due to this bug.

Do you need me to upload worlds again? I think the minecraft scenario “The Mountain” is a much faster repro than the vanilla world. It truly is very quick to show the error. (see other comment). If it’s a matter of “our testing lab for switches isn’t on the eshop so we can’t download that”, I can attempt to generate a more trivial procedure to reproduce the error.
If you believe this error was specifically, intentionally remedied in a version (which I can download on a switch), I will test it.
If you believe this may be related to mesh networking/eero (even though we saw it on simple wifi too), I likely can find three iOS or desktop OS or a chrome OS version and try to reproduce it, but I’d need a little guidance to make sure I had an equivalent version.

And as to eliminating the chance of bad RAM/radiation/high elevation on one of our three switches: We are only about 1000ft above sea level.
We see crashes only in places in other games other people report crashes. We see them all at the same places in games, including when not playing networked games. (Like Dead Cells only crashes at load screens that are widely reported for crashes, or upon specific unlocks which crash other people). Most games never crash. (We definitely understand how unreasonable of an expectation that is for a game the age of minecraft)
We live in a place with low radiation brick, in a brick house (which lowers cosmic ray interference from the side) and almost never have radon in basement’s in our area. We are friends with professional radiation maintenance workers and the last time a geiger counter was in our house there was not incidental radiation (for fun, not because of contamination).
Our Switches are powered through surge protection if not surge protection and battery backup. Power interruptions are rare and uncorrelated with the issue.

I have not done switch development, but, if the switch allows attachment to arbitrary processes by a debugger, and you let me know, happy to do so or setup a VM on a local machine you can remote desktop into and control. If anyone on the MC team is in the Microsoft building on 17th street in Atlanta by Atlantic Station happy to bring down the switches in person to demo it for them (or hell, have a visit at my home if they care, it’s not far).
If I seem overly interested in getting y’all a reproduction: It’s clearly the hard part, both technically and “business decision” wise. I believe this is a QOL issue for switch players which effects “low to medium” revenue customers, the exact ones who are most experimental with “low cost and microtransaction” games. I don’t want competitors to become the default everywhere, etc, because y’all start missing the low spec but still current generation consoles and the like, but I bet whatever changes causes this issue likely advanced some other business or development goal, but not with the intent to disable this capability.

Can I get other players who are on three switches but have problems to try to do the mountain scenario to confirm for the devs how quickly they can generate the same error messages I showed? It’s a truly fast reproduction of the error. Less than an hour.

I’m severely disappointed by this bug and I never would have bought minecraft bedrock edition if I knew it was experiencing such a detrimental bug. Why even have a game released that does not work? Bedrock edition should be taken down until further notice and I want my money back. I have been a long time minecraft player both Java and Bedrock. My friend who is a noob to minecraft and I started a world recently. Simple, easy, vanilla survival world, and single player or split-screen play worked fine. But only a week into the world as the world began taking more MB and as we started connecting using two different Nintendo switches or iOS and a switch it has become UNPLAYABLE. UN. PLAY. A. BLE. I am disconnecting from the game every 30 seconds to a minute. I have searched the internet and have not found anyone reasonably addressing this but I am glad I found someone who is. Our nintendos have over 2GB of space. Our internet connection is pristine and crisp, 5ghz 40+mbps upload and download speed, nintendo online works fantastic with all other games EXCEPT MINECRAFT. The game will lag as soon as I try to open a door or a chest I can tell I’m about to get kicked out because itll open the door or break the block but I wont hear the sound it makes and then oh “disconnected from server” EVERY 30 SECONDS. Please for the love of god give me my money back and take the game down til it works

I've been following this bug for some time, and a recent email about a new comment reminded me that I found a solution, so I figured I'd post it here in case it helps others.
I was experiencing this bug since 1.20 (Trails and Tales), but ever since I bought a Realms subscription this issue disappeared completely. Realms offer a lot of benefits (like automatic world back-ups and letting friends or family play while you can't), but as this is a paid solution, I understand it's not for everybody.

This bug does occur on all devices it seems. From what I have been experiencing, especially lately, the bug seems to occur in the areas that have been massively edited by the player. Could this issue be a data save issue or an issue with how the game loads in the world? If so, it could explain the common lag issues in those areas specifically and why the player keeps getting disconnected from the client. It also could explain why those with great processors and internet speeds still experience this gross amount of lag. The game itself is unable to process the changes properly. I would love for this to be marked high priority. Dear Mojang has been making lots of wonderful changes under the hood so to speak, so I imagine the save and load process might need some tweaking to compensate. I have no complaints if they dedicated a whole update for this fix!