I did a search and looks as though it was has not been brought up on this version.
I have been testing villager carrot farms on creative peaceful, and I have along the line of 25+ villagers in the loaded chunks, I walked away for an hour to test rates and 4 villagers despawned.
I had traded with them to stay with their work stations.
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[Mod] Celesian
Thank you for your report!
We're already tracking this issue at MCPE-21416, so I will resolve and link this ticket as a duplicate.
If you would like to add a vote and any extra information to the main ticket it would be appreciated. If you haven't already, you might like to make use of the search feature to see if the issue has already been mentioned.
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This happened to me as well. A long standing villager disappeared on the farm. Beta 1.14.0.52