Name Tagged mobs despawing

Put the summary of the bug you're having here:

My name tagged Endermite is despawning once I leave the area. I has broken my Enderman farm.

What I expected to happen was...: The mob would stay there because that's what nametagged mobs normally do
Describe what you thought should happen here

The mob would stay there because that's what nametagged mobs normally do

What actually happened was...:  The mob despawned
Describe what happened here

Steps to Reproduce:
1. Put a step by step guide on how to trigger the bug here
2. ... Nametag an endermite
3. ...Walk away