Ive searched but didnt find this issue. Mobs dont despawn when you move to far away from them. I was working on my base for 5 or 6 consecutive ingame days and when i walked to my tree farm i was met by a hord of flaming mobs that wandered into unloded chucks. This ruins mobfarms because mobs spawn in caves below you no mater how high you are and dont despawn.
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Auldrick
Duplicates MCPE-16291.
I suggest reviewing MCPE-16291 and its related tickets for the discussion about mobs despawning beyond a certain distance.
Note that the wiki has now been updated with details of Bedrock mob spawning. There are separate mob caps for spawns on and below the surface, so if your mob farm is on (or above) the surface it can still spawn mobs even when the caves are full.
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Thank you this wiki page really helped me out. Good to see its been updated