I made an underground jail for my important villagers because I hate it when they die. It is fully lit and every villager is in a secure cell with an iron door, in each a bed, a job site block, and a torch. I've been using this for months every day, visiting the villagers to trade regularly (they are all master). None of them ever died or took damage. Today when I logged on and traveled to my underground village, all of them were gone. Every single one.

Villagers still exist in my other villages, one of which was only about 80 blocks away a little further underground. In this further underground village, which I use to breed them, three of the four villagers disappeared just a few days ago, but I didn't think as much of it, because that village is not as secure. I don't know if it is related.

Any ideas? I travel a lot in game, to places many thousands of blocks, or even tens of thousands of blocks away, and change my spawn point frequently, in case there is a known bug related to distance.