I am experiencing this problem in a non-realm world in version 1.16.1

The problem I am experiencing is that villagers constantly despawn even though they are not supposed to. Using a name tag to name the villagers does not fix the problem.

I had a librarian villager that had 2 trades for mending books and it took me about 2-3 hours to make this villager have these trades and I just found out that it despawned randomly. This is extremely frustrating because of the amount of time it took to make this villager (most of the time was spent dealing with other villager-related bugs including workstation linking issues).