Mobs keep disappearing even in small fenced off areas.

I have a cow farm that has despawned cows, sheep that despawn even though they're in fences that allow them 1 block movement, my chicken egg farm in a 6 x 6 area, 3 pandas that were in a 6 x 3 fence, 4 mooshrooms despawned in a fenced 5 x 3 area, and some skeleton horses I just captured today from a horse trap. I never had this issue until the new update today. It stinks because I traveled over 6,000 blocks to bring the pandas and mooshrooms back at my set up area.