I'll try to add more information, we have a room full of villagers with beds but lack of access to their workstations, I was cycling villagers and one of them got mending, I would buy either one emerald using paper or one mending book. after that, I would go back to other projects, either out of rendering distance or out of game, the first time I went offline, the second time I went to the nether, I later got a third mending villager and leveled them up, that seemed to allow him to keep the book. Every time the villagers had slept, I assume that's what allowed them to cycle. Trading with them dosent work, but leveling them up does