Villagers are despawning and locked trades are changing

I’m playing in peaceful mode currently. I had approximately 20 Villagers, almost every trade (locked in and at least Apprentice level), several unemployed and a few nitwits. I saved for the night and the next morning when I logged on the only Villagers I still had was 3 unemployed, 2 nitwits, a Fletcher, Toolsmith, Farmer and 2 Librarians. However, my Mending Librarian (Journeyman level) had now changed to a Novice with a Multishot book. I thought this couldn’t happen? Now I find the unemployed Villagers won’t pick up a trade at all and this afternoon, both the Fletcher and Toolsmith, who was Master Level, have now disappeared as well.