Two villagers that I had in a safe place 15 levels into a mountain, with no windows or exits, were turned into witches. Both villagers were turned and they were the only ones in that room. The room floor was at y58, their ceiling was at y65, and above that was stone, before my base starts at y74. There are no holes to the outside and I didn't have a trident. There was no access to water or Drowned, they were in a stone box with beds, lamps, and a lecturn and smithing table that they could not reach.