Not sure exactly when it happened but I just went down into my trading hall and there was 3 villagers that have become zombies for seemingly no reason. They are all in very well lit areas with access to their beds and their job blocks. They are cured, master level villagers. 

I traded with them very often they all 3 bought my iron. They were two armor smiths and one weapon smith. 

They retained their names and are still named. I have checked every nook there's no dark spots for zombies to spawn to kill them. 

When I cured them back they still were master level and had the same trades. It was just strange to see them safely in their apartments with no way of getting hit by zombies but they were zombies.