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.