As the title says: Villagers who are named and turned into a Zombie Villager afterwards despawns when the chunk it is in gets unloaded.

Provided video (taken in 19w39a, although the issue is also found in 1.14.4 as seen at 7:19 here https://youtu.be/WLygMYnQu4g?t=442) is sped up in order to fit the size limit of attachments