Repro Steps provided by @unknown and comment here:
Steps to reproduce:
Load
. This world contains an OW portal at -30, 68, 0 and a Netherside portal at 0, 54, 2. Near the Netherside portal there is a large herd of pigs trapped in 1x1 space. There are also command blocks set up for toggling the blocks around each portal's coordinates in the oppposite dimension.
Enter the OW portal. You should take damage when you exit in the nether.
Change to creative, fly over to the command blocks near -35, 60, 0, and flip the lever. This should change the stone to glass.
Go through the portal to the overworld.
Change back to survival.
Enter the OW portal again. You will not take damage when you exit the nether this time.
Repeat steps (2) - (6) entering the Netherside portal and exiting in the overworld. The command blocks near 0, 63, 3 toggle the blocks around the Netherside portal's coordinates in the OW between dirt and water. As with the command blocks in the nether, you will take damage coming into the dimension when the redstone torch is off, and you will not take damage when it is on.
If you like, repeat all of the above after killing the pigs. It is likely you will not take damage at all.
Expected results
You never take damage from using a nether portal.
Observed results
When the entrance portal's coordinates in the exit dimension are solid blocks and there are enough entities near one of the portals to slow down chunk saving/loading a bit, then you take suffocation damage from being inside of the solid blocks at the entrance portal's coordinates in the exit dimension during loading.
Original Description:
Both my son and I have recently been traveling through portals. The location doesn't matter. He travelled through the end portal back to the overworld, I travelled through a nether portal back to the overworld.
Both times, we would go into the portal, the screen would change to "Generating terrain" and we would immediately hear the sound of taking damage. In both cases, we lost everything we were carrying. Unfortunately, both times, we had also made extended trips to each dimension. Eventually, the game freezes and we die. When we restart the game, we are at our last spawn point, and everything we were carrying or wearing is gone. When we go back to the portal to attempt to recover it, everything is gone. When my son died after traveling through the end portal (End to overworld), the text said he fell out of the world. No one was around when I died, so I don't know what the message was.
I lost two stacks of nether quartz blocks, two stacks of gold ingots made from nuggets, 18 pieces of nether debris, the shulker box it was in, and all my netherite armor and tools. My son lost a stack of diamonds, several stacks of ender pearls, all his armor and tools, a stack of shulker shells, and 5 elytras.
We'll get them back I'm sure through game play, but that's a fairly serious bug to have happened to different users twice. We are currently running a private server on my synology server at home through a docker container. It's up to date, as we can't join it from our iPads unless the versions match, so it's also 1.17 and change. I don't expect that we will get our things back or anything from the game creators, but it would be cool if we did. Thanks.
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Hi, I just had the same thing happen to me entering the overworld from the nether in Bedrock v1.18.1 on Windows 10. I found a video in this previously resolved issue (MCPE-140759) that shows exactly what happens. https://youtu.be/jleLGCZvbQU?t=372

Another example for the devs.
This just happened to my son yesterday.
Details:
Nintendo Switch (v1.18.2)
Crossing from nether to overworld
Lost all items, couldn't find them anywhere and lost XP as well
Normal sized portal
The loading screen took unusually long
He suffocated
The Totem Of Undying didn't save him
Thoughts:
Playing today my son noticed something odd when he was in a portal in the Nether he dropped a piece of glass and it transported to the Overworld on its own. Not sure if that's related or not he just didn't know if that was a normal thing occurrence.

World Details
Seed Number: -952208073
Edition: Bedrock, 1.18.11
System: Switch
Coordinates of Portals
Overworld: -3334, 24
Nether: -417, 3
Overworld portal is 5x5 whereas the nether portal was standard 4x5.
The portals are both accessible on one side with a wall of stone blocks immediately behind them.
Experience
I died when trying to travel back to the Overworld. Inventory had a bunch of shulker boxes full of sand and misc. items.
Loading screen went on forever and I heard sounds of taking damage.
Eventually the damage sounds stopped (I assume I suffocated some how) but the loading screen never went away.
I quit out and loaded back in. My custom avatar changed back to Steve and all of my items were gone.
I tried digging around the portal in both the Overworld and the Nether, but no items were there.

I'm coming back to update after my son found something useful on the following bug:
In short, my son was able to recover his items by visiting the coordinates of the nether portal in the nether itself. All of his stuff was there and so was his XP. Even after almost 2 months of regular playing.

I tried the above fix as well.
Since I died in when transporting from the nether to the Overworld, I flew to the Overworld coordinates of my nether portal and my stuff was all there. I had to dig down a bit, but the items were in a cave at those exact coordinates.
Nether Portal: -417, 58, 3
Overworld coordinates with those items: -417, 62, 3
Did this happen in a Realm or multiplayer world, or a locally saved world?
If you have a copy of the world available, you might like to upload a copy of it for investigation.
On Windows 10 you can use the Export world feature.
On Android or iOS you need to use a file explorer to navigate to Device storage > games > com.mojang > minecraftWorlds
On Xbox or Switch the only way is to upload it to a Realm, and then download the world on Windows 10 or Android/iOS. (Please do not purchase a Realms subscription just to provide a world to us - but if you have a Realm already this may be a viable option if you wish to do so.)
If the world is larger than 10MB you can upload the world to OneDrive or a similar file sharing site, and then share the link.

Hi, I am having the same issue in 1.19.0. I am playing on a multiplayer server (not a realm). I created a separate ticket because it is in 1.19, but it might be considered a duplicate issue. (ticket linked below).

Ticket from previous comment was resolved as duplicate without appropriate versions being ported to this ticket. I have not tried to reproduce the issue in 1.19.22, but I know for certain this affected 1.19.21

Something similar happened to me a few times. Except it says I was killed by a slime, but I was stuck on the "world generating" screen. Even after I build a crude house around the portal to prevent slimes from spawning. A few times it also just crashed the game. But eventually it would work, but I lost all my XP and some items. Super frustrating to not be able to do anything while "in limbo" after going through the portal (either direction) and be killed by a stupid slime. I'm sure part of it is because my windows laptop can barely handle the game so it is slow to load when going through portals. But surely there's got to be a way to fix this.

Forgot to add info:
v1.19.40
Bedrock
World Seed: -8219632010780896324
Portal Coordinates: 476, 15, 249

This has happened to me 3 times now. The issue seems to be that if you get stuck in the "Generating World" screen when entering a portal, after closing the game and logging back in, you are sent to the dimension you were trying to get into; but with the coordinates you were in. For example, if you are traveling from the Overworld to the Nether at the coordinates 800,-20,1600, your game freezes while generating the world and restart the game, you'll be in the Nether, but instead of being at coordinates 100,5,200, you'll be at 800,-20,1600, and therefore you'll fall into the void.
I noticed this because the first time this happened to me, I was traveling from the Nether to the Overworld, and instead of being at 0,64,800, where the portal is, I was at 0,11,100. I did happen to spawn inside rock and I suffocated while I was there, but I survived because of a totem I had and mining as soon as I could.
The second instance was when I was traveling to the Nether and exactly the opposite happened. I was located at 0,64,800 in the Nether instead of at 0,11,100, forcing me to travel to the portal.
And the third time, I traveled from an underground portal towards the Nether, but I spawned in the void with no way out but hunger documented here.
https://www.reddit.com/r/Minecraft/comments/ystjkj/
What I believe happened to Adam Cox and their son was the Overworld portal was below 0, and they simply fell into the void.
Edit as of 1st-Dec-2022: As a note, I was playing locally on my Switch, but since I passed the save to a realm and I've been playing on PC, the load times are reduced significantly, and I haven't been stuck in the "Generating World" screen for more than 10 seconds since then. Might be a realm thing though, since I haven't attempted to do it locally.

The same thing has occurred to me, ps4 solo local world.
Entering the Nether from a new portal location (480 blockas away from my old one which i tore down) synced to old nether portal location in the nether, and i was getting hit and weakened (or killed) while loading terrain. I tried 4 times somehow manage to survive one time JUST and headed straight back through my portal for safety, a new overworld portal loaded in a random cave near where my old portal was and thus I was killed while loading terrain.
This issue needs to be fixed asap!

hello, this problem had been solved when we reduced the simulation distance to 4, but after yesterday's update (11/07/2023) the problem came back and is now much more evident, I suggest you review the update made yesterday to find out what happened

This is also happening on my Realms server (bedrock). This only occurred after the most recent update.
One of my players has already suffocated to death twice. I'm also sustaining damage whenever I traverse a portal. Not enough for me to die in one instant, but a few hearts of sustained heart damage. Pretty immersion breaking.
I have a video but your limit of 10MB prevents my upload of it.

hello, in our realms we try some possible temporary solutions until the technical team solves it definitively,
problem: apparently it's a possible spawn glitch that causes us to spawn above or below the portal, causing us to suffocate in the ground or receive damage from mobs.
possible alternatives:
1 - break and reconnect the portal on both sides (nether and overworld) to correct the respawn height.
2 - expand the portal to a height of 6 blocks or more.
3 - open an empty and protected space of at least 3 blocks up and down from the portal
additional - always use a totem and a pickaxe in hand for extra security

Added note: said friend has mentioned this only happens during the loading / Building Terrain screen. Given that he's playing on an older console, there is a higher risk of death due to longer load time.

I assume that the recent additition of short sneak has messed with player heights a bit too much. Prior to the update a notable stamping of feet could be heard when exiting the nether, leading me to assume that the player is clipping with something... And now we are taking damage as we suffocate. Death logs support the suffocation, as well as seeing the breath meter. Unfortunately console players suffer more as I've seen them float standing in the portal suffocating as they load the overworld. My last observation is that this only seems to affect realms, likely due to lag.

I have had this happen a couple of time since updating to 1.20.10, both times i was travelling from the different locations in the nether to the overworld and the game just hung on the “loading terrain” screen, I killed my hung game and logged back in, both times it dumped me back in the overworld underground and 400 blocks or so from spawn (in roughly the same location and depth), the first time in a dark cave where I luckily managed to avoid being killed by a creeper and then dug my way up and out, the second time I logged in to being suffocated in stone but was able to mine my way out of it and dig back to the surface.
Update:
After some checking, it appears that when logging back in from the crash, i am being dumped to my last nether coordinates in the overworld.

hello, after update1.20.11 the problem seems to have reduced.
let's test today's update 1.20.12

after quickly hopping onto the realm with 1.20.12 the suffocation persists... 😞
To make your bug report as effective as possible, please try and include the following steps to reproduce the problem:
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(Briefly describe what happens)Expected Results:
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I believe this report was originally describing the same issue as MCPE-158215 and later became conflated with MCPE-42310, but there is a new issue in 1.20.10 that causes usually minor suffocation damage or perhaps sometimes lava, fire, or fall damage. The underlying behavior is related to MCPE-115933 and MCPE-158215 in that the player is initially sent to the entrance portal's coordinates in the exit portal's dimension. In 1.20.10 the subsequent teleport to the exit portal coordinates always succeeds (since MCPE-115933 and MCPE-158215 are fixed), but if it takes too long to load or save the chunks around one of the portals then then the player can be affected by the blocks at the entrance portal coordinates in the exit dimension.
Steps to reproduce
Load
. This world contains an OW portal at -30, 68, 0 and a Netherside portal at 0, 54, 2. Near the Netherside portal there is a large herd of pigs trapped in 1x1 space. There are also command blocks set up for toggling the blocks around each portal's coordinates in the oppposite dimension.
Enter the OW portal. You should take damage when you exit in the nether.
Change to creative, fly over to the command blocks near -35, 60, 0, and flip the lever. This should change the stone to glass.
Go through the portal to the overworld.
Change back to survival.
Enter the OW portal again. You will not take damage when you exit the nether this time.
Repeat steps (2) - (6) entering the Netherside portal and exiting in the overworld. The command blocks near 0, 63, 3 toggle the blocks around the Netherside portal's coordinates in the OW between dirt and water. As with the command blocks in the nether, you will take damage coming into the dimension when the redstone torch is off, and you will not take damage when it is on.
If you like, repeat all of the above after killing the pigs. It is likely you will not take damage at all.
Expected results
You never take damage from using a nether portal.
Observed results
When the entrance portal's coordinates in the exit dimension are solid blocks and there are enough entities near one of the portals to slow down chunk saving/loading a bit, then you take suffocation damage from being inside of the solid blocks at the entrance portal's coordinates in the exit dimension during loading.

Same issue 1.20.12 hotfix (see Minecraft_Port.mp4) - Android One UI 5.1
Player takes damage exiting Nether Portal if standing on a half block while entering the potal.
No damage if player is standing on a full block.

Had this bug effect me just a few days ago when traveling from the nether located at -49, 58, 513 to one in the overworld at -392, -12, 4110. Game froze when loading the world to the point I dropped out, and upon reloading world I found myself at the nether portal's coordinates (-49, 58, 513) in the overworld (under the water almost drowned if not for my armor's enchantments extending my air supply). Thankfully there was somewhere nearby to swim to as there is roughly 3000 blocks of ocean between the two spots.
I am not sure why the game is throwing the player into the new dimension with the old dimension's coordinates instead of loading the new coordinates before placing the player, but it is a really evil way to rob players of their important gear/equipment. I would have been furious losing all my fully enchanted Netherite gear and armor doing what is one of the core functions of the game (going through a portal). Especially with the added difficulty of replacing those items on a older Realms world.
I am terrified of going into the Nether or End at this point on any of my games due to the risk presented by this bug.

Based on my tests, this issue may be limited to portals created in the cherry grove biome, as portals elsewhere seem to function normally.
The world in which I originally observed this behaviour is a vanilla multiplayer server, in Survival. I've been able to reproduce it on a local copy of the world, as well as a different world that used the same seed, was created locally, and has only been played locally.
I also tried a new world with a similar spawn (ie. in a cherry grove biome), and did initially observe some suffocation damage, but was unable to reproduce the original issue in the different world.
The seed for the impacted world is 842595. The other seed I tried was 447.

Ever since the new update, This is happening in a realm I am in. We call it the murder portal. I always die when the screen says “building terrain” I can hear my player suffocating. And I can’t do anything about it since it is still in that loading screen. I die with and without my netherite armor. This was not a problem before the update. It’s very annoying.

I have noticed that the suffocation damage will occur with or without a Nether Portal. It will happen if the player is in the Nether has traveled many blocks away and is teleporting back into the Overworld with the use of command you can clearly see the player taking damage
[media]

the problem persists, but it is causing less damage than in the last update,
we are spawning outside the portal layer and taking damage until the map loads.
I hope the moderators fix this problem soon.

Just did some further testing on 1.20.12, and I noticed that the issue persists under normal configuration, but was unable to reproduce when the Experimental crawling feature was enabled on a local copy of the affected world.

Hello, do we have any news about this case? we will be suspending the realms subscription until the problem is resolved because we are having a lot of losses

it was identified that after the 1.20.14 update the problem continues, but with a much lower lethality, and a low frequency.
I believe you are on the right path

Tested on 1.20.15, issue appears to persist with a dedicated server and client.

This also happens when going from nether to OW, on two different portals in my dedicated server (v1.20.15 running on Ubuntu 22.04.3).
[media][media]
HUGE issue on realms.
everyone is dying in the portals of their connection isn’t perfect! It has to be fixed!!!!

Tested on 1.20.30, issue persists.

After upgrade to 1.20.31, I'm now experiencing this bug moving from the overworld to the nether. Minecraft locks up on building terrain. I've left it for up to 10 minutes waiting for it to load. When I restart the game, I am falling in the nether void at my x,z overworld coordinates. I fall continuously past -32,000 block if I leave myself fall that long. If I have elytra on, I can fly back towards x=0 or z=0 and when I hit chunks that are previously loaded, I receive the message that I fell out of the world and die losing all my gear. This is the 4th time this has happened. Really makes me not want to play any longer.

My game crashed while entering the nether, just hung on the loading screen for so long I had to exit and restart the game, when I joined the game it dumped me at my over-world coordinates in the nether which happened to be mid air in a soul sand valley, wherein I was immediately set upon by relentlessly spawning gastos and skeletons! I am now avoiding nether travel completely until this bug is resolved.

This report is not about the game getting stuck on "Building Terrain" when entering a portal and then being teleported to the wrong coordinates after restarting the game. That issue is being tracked at MCPE-173386.

This issue has not been resolved and it's still happening on my world! 😡

Now 2 things are happening
Still taking damage when “building terrain” during portal travel. By chance you survive that and not suffocate in wall then 2. happens.
during “building terrain” the game gets locked in that screen and you have to log out of the game. When loading back in, it will load you back in at the coords of that portal but in the opposite world. And usually everyone dies

I haven't had much problem with this while playing on PC (no more than a heart or two of damage and only sometimes) whereas playing on iPad has caused a couple of deaths while going through nether portals. Recently though, playing on PC and 1.20.41, I switched out some blocks in front of a nether portal (full blocks both before and after) and afterward died multiple times by suffocation (both going from overworld to nether and the other way around). It's not the only change I have done but the only change close to the portal.
The portal on the nether side was 2x4 and the portal on the overworld side is 3x5. After changing the nether side to be 3x5, the problems stopped.

Hello darling players and mods. Just wanted to add my thoughts about this and hopefully help create a solution. One thing I noticed was that wherever I ended up in the terrain, I was always surrounded by gravity blocks (aka sand or gravel)
PLAYERS! I am someone who typically plays on Peaceful difficulty. Because of this, I have managed to avoid dying by suffocation as saturation on peaceful is always maxed and I'm able to heal, giving me time to get my pickaxe to dig out the block that is suffocating my head. PLEASE CONSIDER switching to Peaceful before returning to the Overworld via portal!! I PROMISE it will save you!

I’d like to add that I was able to resolve this issue on my realms by simply changing out the world and moving the original on a new world slot. I also found activating another world and then reactivating the original at a later time works as well. I no longer take any damage from entering nether portals.

Issue persists in 1.20.51, on my realm. I suffocate before the loading screen finishes. And end up seeing my stuff scattered in an empty field, as if I simply suffocated in the portal itself. Peaceful mode and everything, and I still have no chance. Good to know this bug is months old and hasn’t been fixed.
Edit: further bug research shows this issue isn’t just on the, but YEARS old. Yeah, I doubt it’s getting fixed any time soon at this rate despite it being a borderline playability issue.

I'm currently playing on a survival world 1.20.51 on my Xbox One. Today, I died while traveling through a nether portal to the OW. During the loading screen my game stopped loading. Therefore, I closed the game and reopened it. I loaded up the game to a death by suffocation. All my stuff was gone. Checked both sides. This isn't a new portal and I've traveled through it more times than I can count. This is the first time that I've been killed in this manner.
I've only started to research this bug as I'm a return player. Like others, I'm quite frustrated with this issue. Hoping for tips to get around this or a full solution. Thanks.

Can we please get an update whether this is considered in the fix backlog? As the worlds become older this is becoming more and more of a problem especially on the servers. We had to start removing some mods to speed up the loading and stop players being killed by the portals. Seeing the number of votes and continued attention we'd like to get this escalated for attention by devs.

This is a major problem. It should not be difficult to portal travel. Before I would suffocate in wall during the building terrain screen, and I would die, but my stuff will be right at the portal easily to retrieve. Now I get stuck in the building terrain screen, forcing me to log out and to log back in with me dying. Somehow stuck in my inventory is located at the nether portal coords but in the overworld. Luckily my teammate was able to go to the overworld with the nether portal coords I provided and was able to get my stuff back. Please fix this problem. This is an a month old realm and I play on iPad. But realm players

With the release of 1.20.60, and the resolution of MCPE-173386, the two servers I play on has stopped experiencing this bug. Can someone do some testing, as we believe both of these bugs were caused by the same issue.

I cannot reproduce this issue in the current version.
1.20.62 Hotfix was added to the "Affected Versions" list by the resolution of MCPE-178993, which seems to describe this occurring on a realm. If you are still experiencing this bug, could you please provide more information about the circumstances in which it occurs?

This just happened to me on my realm today. I'm playing on Pocket Edition v1.20.62 - I built a large nether portal in the overworld (7 blocks wide x 13 blocks high) as I'm making it look like an old stone archway in front of it.
When I first built the portal I heard a piglin dying in the portal (not kidding, it was screaming) which I have never heard before.
This was my first time going through, I was near the outer wall of my Overworld portal, I could hear that I was suffocating on the loading screen and then I just died and respawned.
I went through again and stayed more in the middle and I was able to get through and pick up all my stuff.
I got a lay of the land, picked up the quartz I needed and went back through. No issues going back to the overworld. I haven't tried again, but after I came out, about a second later I heard a piglin dying/screaming again. My portal in the nether seemed very high up in the world, about 20 blocks below bedrock ceiling. My portal in the overworld is at 207, 123, -23 and my nether portal is at 39 ,104, -18

The piglin dying that JadeFardreamer describes is most likely due to the portal being built against a wall of solid blocks. Zombified piglins spawn randomly from portal tiles, and they always spawn on the south or east side of the portal. If there are solid blocks on that side they will spawn inside them and suffocate.
I am not sure what to make of the player suffocation on going through a portal for the first time. If we could get the seed along with the coordinate we could possible test that issue. If it was actually some other type damage, such as lava or monster attacks, it could be due to MCPE-42310.
I believe the original cause of the damage reported here is fixed. There is a new issue with taking suffocation damage when going through a portal that is built in areas that have been modified by the player, which results from a combination of MCPE-169059 and MCPE-160998.

Not sure if this helps, but I noted that on going through portals from the overworld to the nether, my load time was significantly longer than it should have been, and upon loading the destination I was frequently finding myself in the right location but the chunks around me appeared to have been reset, initially I was worried my world had been corrupted, but then after some time the chunks finally loaded and my nether builds appears all around me. It got me to thinking that if this is happening in a place that used to be solid blocks, could it potentially cause suffocation and death?

Sadly i experienced this problem like about 2 hours ago, i hope it will be fixed before or when 1.21 comes out..

Happened to me in 1.21.10, in a realm

While loading from the nether to the overworld in 1.21.1 it took about 30 seconds to load, then it dumped me inside the obsidian portal blocks! Luckilly I was able to mine my way out!

My friend on the realm literally gave up on minecraft because of this bug. We were playing on the realm

My cousin experienced a more serious version of this where, instead of taking damage, the entire biome the portal appeared in reverted to its original state, taking the portal along with it. We had built a bunker that the portal was contained in that ceased to exist for a few seconds.
[media]Reply from @unknown:
The issue you're describing is being tracked at MCPE-169059.

the bug it is still occuring on the latest version on realms

Happened twice to me in my realms session today. Once when portaling from overworld into the nether, resulting in me falling into lava (lived due to totem of undying), and once portaling into the overworld from the nether, resulting in near suffocation (was able to dig out). Extraordinarily inconvienient bug, definitely NOT resolved.