Villagers are all zombies but there is no zombie

I have a house full of cured zombie villagers. There is an iron door and that is the only way in. However, when I came back from fighting phantoms, the villagers are gone and there are a few zombie villagers. I checked for any dark places or any openings but there are none. Every place is lit up so no mobs can spawn. The floor is also made of lower half slabs. There is a glass dome but no opening. Please help. I have used so much resources on them (more than 10 villagers).