Disappearing Horses & donkeys

I noticed this in my survival world that I started when 1.10 was released. I caught a horse and a donkey and named them and put them in a pen. Then I took the donkey out for a ride and about 1/3 of the times I dismounted it, it would disappear. I had to log out and relog into the world and the donkey would reappear. Later my named horse disappeared after I logged in. I was staring at it when I logged off and now it's gone. Same thing happened to my donkey, I relogged but alas, it's gone for good...

What I expected to happen was...:
Have mounts that didn't disappear 1/3 times I exit the game.

What actually happened was...:
Exactly the opposite

Steps to Reproduce:
1. Look at an animal
2. Exit game
3. Reenter game
4. 1/3 (my luck) of times, you lose an animal. Named ones too