Sometimes when I go to the nether, log out and log back, a levelled-up Villager (Master or expert) has lost his profession and claimed another workstation. This leads to either changing their profession or changing his trades.

Note that these villagers that I encountered the problem with are :

-Old Villagers(may be not important)

-Don't have a workstation.

-Or can't find a Workstation.

Examples:-

I had a farmer who was named Faromeo and he changed into a Cleric.

I had a librarian who changed his trades(he had Frost Walker I and Blast protection IV and then he has now (LotS I and Blast Protection I))