Villagers I've traded with and leveled up revert to novice

Multiple times, I've had villagers I've traded and leveled up with lose all their experience, revert to novice, and change their trades.

First, it happened with a shepherd twice, and now it happened to my librarian.
No other villagers had their trades change or lose any levels at the same time, so I know it wasn't two villagers losing access to their work stations and trading jobs, and I didn't have any unemployed villagers at those times, so it couldn't have been them losing access to their work stations and an unemployed one taking over.