Lost names of villagers

So I named my villagers because I do not want them to despawn. After some in game days, their names were gone. I do not know what happen. Luckily it was just their names and not themself. But I'm afraid since they are unnamed, they are probably going to despawn and I do not want to waste my time and emeralds and iron to name each of them again and again. Please fix this. Besides, I do not think villagers should despawn. Im not sure if this bug happens to other mobs too.