In my world, any mob type are randomly despawning. I have a cow farm, and a leatherworker right next to it. They all despawned, even if I'm in the same chunk or not. It has happened multiple times before, but mostly villagers since I work with them a lot.

Edit: I counted my sheep, and only found 15 instead of 16(I have every color of sheep). My orange sheep had despawned.