Two villagers despawned while I was standing next to them. They were librarians and had already been traded with (their trades were paper and each a fortune II or protection IV book). No hostile mobs were around and no damage noises were heard, so they cannot have been killed. They were confined to a 1x1x2.5 block area for trading purposes, with one free block to interact with the player at head hight and their workstation underneath. World and villagers initially generated in 1.18, unable to reproduce the bug in testing, the bug happened only after having them loaded in the world for a long time.