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
Set difficulty to hard
Spawn a villager
Name the villager with a name tag
Spawn a zombie
Let the zombie infect and convert the villager
Run
/tp @s ~ ~255 ~
→ ❌ The zombie villager despawned
Related issues
Comments

This happened to Mumbo Jumbo in his recent Hermitcraft Episode, So it's confirmed by him and all the viewers.