I have a small trading hall I have built underground filled exclusively with zombie villagers trained from a nearby spawner and then cured. Randomly the libraria s only have been disappearing. There has been no nether travel before their disappearance and they are also all ranked up to max level through trading. It appears to happen at bedtime. Unable to attach screenshots but all villagers are surrounded by solid blocks, bed, workstation, and iron bars. There is a small gap left so that interaction and xp collection is unimpeded. I have browsed the other issue you may insist on attaching this to, but this literally began the day of the 1.16.100 patch. This was not occurring before the update and as mentioned previously appears to be limited exclusively to librarians. The cured villager trade seems to have been adjusted as well, if that is the case they were messed with and need fixing.