Portals crossing chunk border only partially lit

This portal runs along the Z-axis. -369 and -370 are both portal blocks, while >= -368 are simply obsidian. Using a chunk calculator, I see that this is clearly a chunk border. Is this expected behavior along a chunk border?

Does the same behavior occur on a downloaded copy of the world?

It does not. My server owner downloaded the realm locally, broke the portal, relit it, and upon coming back out of the nether, the portal was still intact.

EDIT: server owner has confirmed that the portal IS still breaking as detailed in my post in an offline world.

Thanks for answering the questions. If you try to use the portal on the side that is "broken" does it still work as a portal? As in, is the effect just visual? Does it matter which block you light the portal on?

Edit: I'm going to request to move this to the "MCPE" project since it is apparently not specific to REALMS, so thank you for that additional information.

Thanks for redirecting to the proper team.

To answer your question related to the portal: No, it does not. When the portal is "broken", I can stand on the unlit obsidian and it acts exactly as unlit obsidian; no particles, no teleporting to the nether. After breaking and re-lighting the portal, it does not matter which block is "lit", the result is the same–upon re-entering the overworld, the portal is in it's "broken" state where only the two blocks to the left are lit, and the others (in a separate chunk) are not.

Interestingly, I never return to the overworld standing on one of the actual portal blocks, but, rather, always on a normal obsidian block.

I'd like to add some additional commentary to this in the event that it's helpful... The overworld portal runs along the Z-axis, intersecting 2 chunks; the chunk border being directly between the lit and unlit obsidian blocks. In the nether, the portal runs along the X-axis not intersecting chunks in any way.

Hope that helps in some way.

It appears the same thing is happening here as can happen with beds that cross chunk borders. (MCPE-31389) Where there is loading delay/lag, a chunk with half the portal loads first, does not detect portal tiles in the neighboring unloaded chunk, and then breaks. Just like with beds the bed head needs to be able to detect a bed foot or it will break, so with portals, portal tiles need to detect a connection to a portal tile at the saved location of the portal, which is its bottom north or west coordinate.

@Wayne W could you provide the full coordinates of your bottom north portal block, and also the coordinate you spawn on when you go through and it breaks?

The northern most block on the bottom platform is located @ 950, 67, -370. After relighting, entering the nether, and reentering the overworld (thus breaking it again) I spawned on block 950, 67, -364. 

I've reproduced the broken portal several times.

You can see in the above pic that I used Wayne's coordinates and copied the layer of blocks around it from his pictures. I rebuilt this in order to test whether the break could be triggered by the wall and fence blocks next to the portal blocks trying to connect to the portal blocks or updating their configuratoins as the chunk loads. However, that did not pan out. I was able to use the portal successfully many times.

What broke it was relogging with increased render distance and simulation distance. I tried relogging with increased random tick speed too, but that did not cause a break (just lots of zombie pigmen 😛)

EDIT: Re: The position where you spawn coming through the portal:
This doesn't seem to depend on breaking per se, but instead on where the portal in the other dimension is located relative to the portal you are coming out of. In the test world (

), if you enter the northern half of the portal in the nether you'll come out at 950, 67, -370, and if you enter the southern half you'll come out at -368 <= Z <= -364. It's when you come out in this area that the portal can break, because you are loading the southern part of the portal first and those portal tiles cannot detect portal tiles that connect to the saved portal location in the other chunk before it loads.

Is this just a visual issue, or does the portal break completely (meaning it can't be used)?

This issue affects the usage of the portal. Only the obsidian that appear as portal blocks can be utilized—the standard obsidian blocks do not act as portal blocks when attempting to enter the nether.

@@unknown it's a complete break. This is the exact same chunk-border chunk-loading related breaking that occurs with invisible beds (MCPE-31389) and cocoa pods (MCPE-67479). See my ealier comment above.

You can get this to happen more reliably by having powered redstone dust next to a portal that crosses a chunk border. You then load it from the side the redstone dust is on. It gives a block update upon server restarts which updates the portal and breaks it, because the other half is unloaded.

Part of the issue is that portals can be updated from the side. If you stop this behaviour, this bug should stop happening.

Similar issues with redstone torches and signs were fixed in the exact same way. Those blocks only process block updates from certain directions. (sidenote: grindstones can be updated from all sides, maybe fix that too)

However, this is all part of a much bigger issue where instant updates can happen to blocks even if the survival of those blocks depends on things in unloaded chunks. Fixing that may be a good idea.

Also, this affects the 1.16 betas last time I checked.

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.

Here is a world that demonstrates breaking nether portals, beds, grindstones, and cocoa pods on chunk borders with block placement commands.

Here is the world where I rebuilt Wayne's setup and reproduced the bug a few times by setting render and sim distances to max and relogging. Today, however, it is not cooperating.

It seems that the reason I could not get the portal to break again in my previous comment is that I had reduced my Minecraft window size in order to try to capture a video of it happening. Reduced window size must reduce the processor or RAM access enough that there is not enough delay between chunks loading to break the portal. Today I reproduced the break again in full-screen mode with render set to 44 (my computer's max). World sim distance was set to 4. You'll notice in the video below that I have a lag machine running in the chunk where the portal breaks (lots of items perpetually moving in a circle), however, this lag machine was not enough to trigger a portal break with Minecraft running in a small window.

Computer specs:
Intel Core i5-6500 3.2 GHz
8 GB RAM
NVIDIA GeForce GTX 750 Ti

Are there any known workarounds to the issue?

If it is a chunk border issue, possibly the error only occurs when a portal is placed across chunks. Would a portal placed entirely in the same chunk NOT experience this problem?

And if so, would this be true only if the obsidian blocks are also in the same chunk, or would it apply to only the portal blocks?

@Johan Verstergaard Paulsen: yes, a portal that does not cross a chunk border will never have this problem. If only the obsidian of one side is across a chunk border then I think the entire portal could break.

Can confirm this as its happening to my big portal the entire time.
a single block update can fix this when you relight the portal