This bug is an annoying bug that I discovered by watching Mumbo Jumbo's video.

He showcased the bug around 6:57

https://www.youtube.com/watch?v=WLygMYnQu4g

How to reproduce:

1. Get a Villager 

2. Rename it.

3. Let a Zombie convert it into a zombie villager.

4. Fly 128 or more blocks away.

Expected Results:

-The Zombie villager will still be renamed after it was converted from a Villager to a Zombie Villager, but it still despawns.

What Should happen after fixing the bug:

1. Fly away 128 blocks.
2. Return Back.
3. The Zombie Villager is still there because it is renamed.

Hopefully, this is helpful and the bug gets fixed.