Mobs Despawning

Put the summary of the bug you're having here

What I expected to happen was...: 
when i have given my zombie a name i expext it to stay there 

What actually happened was...:
i have wasted 3 nametags on making a ironfarm i also tried spawning them with eggs but they keep despawning

Steps to Reproduce:
1. get Zombie
2. Give name
3. wait and see