Zombie Villagers have their "CanPickUpLoot" NBT tag changed to 1b when converted to Villagers, of course. However, the issue unfolds if the same Villager gets converted back into a Zombie Villager. They will then receive a new, random "CanPickUpLoot" tag. Instead of converting back to the old one. I'm not sure if this is intended, but it doesn't make sense that the "same" entity (keeps some data such as "CustomName" i.e) should change that tag when converted.