The bug

If you name tag villagers and they get converted to zombie villagers they will de-spawn even with them being named when the player goes 128 blocks away from them like normals hostile mobs do.

To reproduce

1. Set difficulty to hard
2. Spawn a villager
3. Name the villager with a name tag
4. Spawn a zombie
5. Let the zombie infect and convert the villager
6. Run /tp @s ~ ~255 ~
   → The zombie villager despawned