Mobs with nametags disappear randomly

Every time I get a villager from another village I turn them into mending villagers. But both times after a good amount of days of struggling to bring them to the hut, they dissapear. I don't know if this has happened to anyone else, but it stinks after struggling to get a villager and it just vanishes. I feel like it's because I named the villager, because I named a sheep and it disappeared after I rejoined my world.