Villagers are disassociating from job sites right next to them and their beds. With no trading (0 trading XP), and without moving their their job site, they are becoming locked to their professions and other villagers can use their job site to acquire their profession.
Not sure how to replicate reliably. It may happen when a child villager becomes an adult when not loaded, causing it to steal the job site. I haven't tested, though.
Related issues
Comments
migrated
migrated
I just had a more severe version of this happen. At least I think it's the same bug. I had a Librarian who I traded with to lock in his offers. I left and came back, and he was now offering a bookshelf. He still had experience points, so it wasn't a fresh job, but he'd rerolled his offers somehow.
GoldenHelmet
We can only track one issue per ticket. It sounds like the primary issue you’re reporting here is new villagers stealing job sites from older villagers. That’s already reported at MCPE-43071, so I’ll have this ticket resolved as a duplicate.
GoldenHelmet
Just a follow-up after looking at your report again--the librarian changing his trades sounds like a version of MCPE-62080. It can happen in similar circumstances to MCPE-43071.
Here's a weird addendum: Sometimes these villagers will continue to function normally. They won't be "assigned" to their job site but they'll still use it. But sometimes they won't, and their inventories never refresh, making the villager effectively nonfunctional.