I've played in three different worlds now and all the villagers despawn after around 6 hours of playing. I've spent like 18 hours in 3-4 different worlds just for my villagers to despawn. They're not moved away from their original spawn point. I breed them and after a while they all just disappear at once
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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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