This bug is an annoying bug that I discovered by watching Mumbo Jumbo's video.
He showcased the bug around 6:57
1. Get a Villager
2. Rename it.
3. Let a Zombie convert it into a zombie villager.
4. Fly 128 or more blocks away.
-The Zombie villager will still be renamed after it was converted from a Villager to a Zombie Villager, but it still despawns.
- 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.