the villagers keep despawning

i have a minecraft world in which i am really succesful. the other day, wishing to get a mending book, i went for my mending villager but he wasn't there. i thought i must have left some gap for him to escape but i found no such gap. after a few minecraft days, my armoursmith villager, protection enchant villager and efficiency villager had also mysteriously disappeared. it seems like they are all despawning. there is no other explanation to it