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:
Fly away 128 blocks.
Return Back.
The Zombie Villager is still there because it is renamed.
Hopefully, this is helpful and the bug gets fixed.
I am sorry, I searched but I couldn't find the bug and I didn't know it was a duplicate.