I am using a mod that add's more zombie villager igloo variants for each biome, but I'm assuming this can be replicated with the vanilla igloos as well.

Issue is simple instead of having zombie villagers in this structure like vanilla minecraft, they are already normal villagers.

This is normal MC behavior. They spawn a villager next to a zombie. Closing this as invalid.

Well my apologies, I was unaware, but there's still no zombie villager though. In the screenshot you can kind of see it's still empty. I might test it with the MCA mod by itself one day.