I have experienced times when mobs seem to despawn when I load into my world. Even if they’re enclosed in a small pen. I’ve even had named mobs despawn such as a mule and lamas (both had been leashed to fence posts).
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[Mojang] Mega_Spud (Jay)
Thank you for your report!
We're actually already tracking this issue at MCPE-21416, so we will resolve and link this ticket as a duplicate.
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It would appear as though this is a universal issue as I have had it on pocket edition and have met people who've experienced it on win 10 as well. The sooner they fix this the better.