Despite having been assigned a job from a lectern, librarians who have reset their trades using said lectern will at random become unassigned and incapable of resetting. Using a machine I've built, I've been putting zombied and cured villagers into 1 by 1 cells then placing a workstation beneath and ahead of them which they detect through a decorative trapdoor. They will then claim it (shown by becoming a librarian), and once I've cycled to a trade of my choosing they are traded with, mastered and watched to interact with their station. I've seen it happen 4 times now where villagers I've brought in that I've watched reset their trades at a workstation next to them, lose it and not find it again. They must be "losing" it because the next villager brought to the area becomes a librarian before I place down their station. meaning the nearby ones have become unclaimed.