Before the 1.16 update, this wasn't a problem. But now, on my Realms server, there is a massive issue with farm animals despawning. In the last 3 or 4 days, over 50 chickens have despawned from a 10x13 pen. About 20 cows have despawned from a 10x13 pen. 6 pink sheep despawned from a 7x7 pen, 5 blue sheep despawned from a 7x7 pen, one turtle despawned from a small aquarium.

I noticed this can happen even after just logging out and logging back in. Today, I logged out, logged back in, and my last pink sheep disappeared.

About a week ago, my friend's carrot farm lost half its carrots and the ground became untilled despite being properly irrigated. Not sure if it is related.

Although it's not new for mobs to despawn by bug, this has never been so severe until the Nether update. I think the current issue is related to the Nether update.

Update: 4 baby turtles despawned before they could grow up. The only one that didn't despawn was one I manually watched the entire time it was there.