villagers disappearing for no reason

I have had problems with villagers sometimes disappearing for no reason. I have had this happen on a server and on private worlds. the villagers in question are well protected and are in a base with an iron golem, but When I log off and come back the next day. One of my villagers is gone. This happens rarely. In my private server the cleric disappeared, in my private world a librarian with mending book disappeared. I don't know if this happens to anyone else but it has happened to my several times.