Since I have an Elytra and slide back and forth it sometimes happens that peaceful mobs or npcs disappear when the chunks are loaded. In the versions, it first appeared for me at the beginning of the year.
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I know, a Horse with armore a villager and a little Bit other animals in a longer Period.
For example, a villager has disappeared underground in a farm. The one who lived with him was alive, so they weren't zombies, maybe someone can take a look over it?
Note that naturally spawned animals and monsters will also despawn at certain distances unless they are persistent (name tagged, or somehow interacted with).