I had all villagers enclosed with beds and trade blocks in their room with a lantern. Most had been converted from zombie villagers from a nearby zombie spawner. One randomly disappeared, but the rest were all there until recently when all of the other ones disappeared seemingly at once. The zombie spawner and the villager area is separated by an iron door, and each villager is contained in their own room. Approximately 15 villagers disappeared. Rooms are well lit.