Mobs that should not despawn. randomly despawn

On one world i had a collection of tamed horses /donkey /mules (fenced in) then for some reason i don't know how they have despawned, samed with some cows i had in my fenced in farm. so i know they have wandered away. 

 

in another world i had a I had an iron golem that just despawned the lead knot was still there but he just disappeared, with another (again fenced in)

 

I wonder if this is related to my other bug that i reported with the crash due to stock overflow error because i had a case when i was riding a minecraft when it crashed that just despawned.