It happens randomly, when I get in back to a world where I have my animals such as horses, parrots, wolves. I'm not certain in villagers and other mobs but tameable mobs surely do.
The steps to replicate this could be:
Create a survival new world.
Tame some animals.
Log out the world.
Log in again.
Not all time that one logs in back the mobs would despawn but it is very likely to happen.
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[Mod] OcelotOnesie
migrated
Thank you for your report!
We're tracking this issue in MCPE-144208, so this ticket is being resolved and linked as a duplicate.
If you would like to add a vote and any extra information to the main ticket it would be appreciated.
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