All Zombie Villagers become "Plains" villagers after converted.

Picture "ss1.png" is a zombie villager spawned in a desert by a spawn egg.
After converting the zombie villager to a villager via weakness potions and golden apples, they become a "Plains" villager, despite their being in a desert biome (picture "ss2.png").

This is caused by zombie villagers having

{VillagerData:{type:"minecraft:plains"}}

in their NBT.