All mobs despawn in loaded chunks upon re-entering a world

This happened twice already. Yesterday I left the world at my base and today I entered the world and found out all my farm animals and horse were gone, all of which were fed and the horse was tamed (which means they shouldn't despawn).

What I expected to happen was...:
I expected the farm animals and horse to be there when I enter the world

What actually happened was...:
They weren't

Steps to Reproduce:
1. Have despawn-resistant mobs in loaded chunks (tamed, fed or spawned by breeding)
   1b. It's possible you need to stay in the world for a while, maybe unload the chunks and                   reload them
2. Exit world
3. Re-enter
4. Witness loss
5. Conduct a memorial