Villages in the end and nether are not saved correctly. As a result they get recreated with each relog and therefore the POI links are unstable. (The original report below focused on world corruption due to duplicate data village data being saved, but that could not be verified in-game. Players have also found that visiting the overworld coordinates matching a nether villager causes the duplicate village data to be cleared: see this comment.) Code analysis has been provided in this comment.

Steps to reproduce

1. Place 2 beds and spawn 2 villagers in the end, then set time to night so that the villagers go to bed. Or use MCPE-85954 Nether Village.mcworld where this has already been done.
2. Relog.

Expected result

Villagers continue to use the same bed they had linked to.

Actual result

The villagers re-link to the beds and sometimes do not link to the same bed they were sleeping in when you saved & quit. To reproduce this, it may help to open and close a different world before reopening the nether village world in step (2). See Nether village unstable.mp4

Original report

How to reproduce:

1. Put a bed and villager to create a village in the nether or end.
2. Log out and look at the village file in universal Minecraft editor and make a note of the villager id number.
3. Now log back into the game everything should seem normal.
4. Log out and go back into universal Minecraft editor and look at the village data again.

Expected result:

Nothing should have changed and everything should be the same as the last time you checked.

Actual result:

There are now 2 villages both with 1 villager and 1 bed. The villager ID number is the same in both villages. Every time you rejoin the world a new village is created increasing your world size. These villagers arent ticked and are just ghost data. You can test this by looking at the ts timer in universal minecraft editor.

Extra info:

Villages in the end and nether can cause corrupted files.
This video explains it in more detail:
https://www.youtube.com/watch?v=jOJbuXctTWY&t=19s

This was the case until the 1.16.0.66 beta where it got worse. Like in before this the files still corrupt and when you remove the village in game it still creates a ghost village but if you dont remove the village it creates another ghost village every time you relog.

This video shows you the issue and how to reproduce this:
https://www.youtube.com/watch?v=jpKbooL43Ns