I was playing the game normally when (i don't know how) a Villager (from my Personal Village) appeared in the Nether. When I travelled back to the Overworld, I noticed that 3 to 5 of my Villagers - Librarians especifically - simply disappeared.

Some of them (who disappeared) had already made trades with me, and the other Villagers from another careers were unaffected.

This Villagers are inside an area where they can't go out.

Note 1: Today I've noticed that 1 Farmer Villager disappeared too.

Note 2: I remember that some chunks dind't loaded properly on that occasion (probably because I entered and re-entered the Nether a few times).

Note 3: I was playing on Singleplayer and with the Multiplayer turned off.

Note 4: When this happened, the remaining villagers didn't entered in love mode to fill the "extra" beds (the ones from the missing villagers). I had to take the beds off and put them ONE BY ONE. Otherwise they just entered in love mode once, but in the end they didn't spawned any baby villager.