Villager randomly turned to drowned

I had a group of bred villager in a lighted fenced area in mine carts. I did not leave the area (chunks) but looked away for a minute and one of my librarians had turned to a drowned. It was on land the entire time and in the mine cart. I lost my mending villager because of this bug. There was no way for a zombie to spawn or enter the area so idk what spaghetti code caused it.