Mobs with Nametags

Put the summary of the bug you're having here

What I expected to happen was...:
I spawned in a named skeleton wearing armour with a command, thinking it wouldn't despawn because it was named.

What actually happened was...:
It disappeared whilst I was working on another project in the same world. Strangely enough, I had spawned in another one 5 minutes earlier and that one is still in the world.

Steps to Reproduce:
1.Create a new world on superflat creative with cheats enabled.
2. Spawn in a named skeleton wearing armour using a command block.
3.Do somthing else in the same chunk for a while and it's likely to dissapear.