Started playing my server after creating a backup. I built for some time, then noticed that all my animals are gone. There were 'wild' fish, but the fish in my aquarium disappeared and so did my horses, sheep and cows etc. My builds are intact. Hostile mobs can be found. I've been playing this world for over a year so I don't believe it's a despawn issue.

I don't know if this is information in helpful or not, but for some reason my Minecraft went from 20w51a to 1.16.4. I read that 20w51 is a snapshot and I don't have them on so I don't know how that's even possible.