I have had 3 villagers just up and vanish even after trading with them on peaceful mode all at the same time. I have been player in the world for days with no problem and then all of the sudden no more villagers and one was at master level. I have also had the same thing happen with chickens and sheep. It is a pain to hunt down sheep of different kinds just to have the disappear.
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Thank you for your report!
We're actually already tracking this issue at MCPE-21416, so I will resolve and link this ticket as a duplicate.
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This bug is horrible and has been happening in Bedrock for a long time, But in my world the only villagers who disappear are those who are in borders of chunks.
All animals and villagers who had interacted with you will despawn. i lost 9 bees and 6 farms because of this bug. It affects the 1.16 versions as well, 1.16.0.61 to be more exact. this bug is old. they didn't fixed it in months.