Villagers randomly disappeared

I have a villager pit and I went to go get a composter and some beetroots but when I came back my all of my villagers were gone without a trace! There was no way a zombie couldve gotten in because it has an airlock of iron doors (and it was day and properly lit) and even if a zombie had gotten in I am on hard mode and the chunks were loaded so the zombie vllagers wouldve still been there (also I checked on them a few minutes before and they were all there).