Nether chunks not loading or get lowered into void

Windows 10 Beta - 1.17.30.24

Flythrough video of same issue: Link

Can confirm presence on 1.17.30.24 on Android.  World seed = 1024

In addition to the strange altitude, some chunks are absent from the beginning, there is a lot of bedrock floor in the chunks that are loaded. and the nether degenerates every time you enter it - the nether-side portal despawns (after 2 successful returns in my case), and more and more chunks are denuded to bedrock or completely disappear.

Also, breaking the overworld-side portal and reassembling it achieves nothing - I built the new one at 90° to the old one(I was trying to force generation of a new portal in the nether, as the old one no longer existed).  I just entered at the same site as the now nonexistent old one and fell onto the bedrock.

There is no "invisible" portal present - I built up to the old nether-side portal site to check for this and could not reenter the overworld.

I concur that the cause of this is probably associated with MCPE-139665, if only because  of the identical death message after falling off a chunk:

Can confirm. Also can happen in overworld, found this video.

Welcome234, the overworld issue in that video looks very like MCPE-139669, where a world created before 1.17.30.23 is converted to a 1.17.30.23 Caves and Cliffs world and unexplored chunks are raised. 

I couldn't see gaps into the void as present in the nether on the current beta or below y = 1 on 1.17.30.23 overworlds (MCPE-139665).

Workaround for this problem on 1.17.30.25:

This issue seems somewhat mitigated on this version (an old world I opened as a test had no nether derangement or nether-side portal despawning despite my entering and leaving the nether more than 10 times)

Doing the following permits stable entry and exit to the nether in cases where the portal in the nether has despawned:

Collect sufficient obsidian for a nether portal and make a flint and steel.

Take these through the overworld portal (along with weapons).

Don't bother trying to reconstruct the old portal.  Build a new portal in the nether in an accessible place.  Activate it with the flint and steel.

This portal will take you back to your overworld portal.

The setup I made(see screenshot below) was stable for 20 passages - entry to the nether was still  through the despawned "ghost" portal, but the new one reliably took me back.

Although I perceived no further degeneration of the nether, the missing chunks(now bottomless pits) had not been restored, making exploration very hazardous.  The only resources I could find were gold and quartz, and I have yet to find piglins, hoglins or blazes, so mine may not even be worth exploring.

I will keep the thread updated on the stability of the nether chunks on both worlds mentioned.

With respect to the state of the nether on the world with the workaround and the updated 1.17.11 world:  No noticeable deterioration, but the nether in the former is in such a bad state that visiting it is pointless.

I would only use the workaround I gave above for worlds whose nether is recognisable (i.e. not become a punched-out bedrock floor with very few biomes remaining) and can be safely travelled through.  The others I'd start over.

This bug also affects the end. The end on my old creative world was shifted down 64 blocks.

This has happened to my game. Can’t really do anything in the nether. It’s like a bunch of floating islands and no lava pits on the ground etc. No fortresses or anything. Just a really messed up nether that we can’t do anything with. 

This bug also affected Flat World.MCPE-141727 was reported that hours ago.