Zombie villagers spawn from zombie villages can despawn(PersistenceRequired:0b)

Zombie villagers spawning on zombie villages can despawn (except plains zombie village).

To reproduce, use the seed -298379770968479212 and use the following command:

/tp @p -2339.5 71 -307.5

to teleport to a zombie village. Notice that the zombie villagers will despawn if you move too far away.