had a librarian selling loyalty 2, checked back on him a bit later and he was selling silk touch, then checked back on him even later and he's selling emerald for paper. it is the same villager as he is enclosed. this happened with a toolsmith aswell. it may have something to do with the fact that unemployed villagers are entering the house and also becoming employed by a job block that is already in use, which is not supposed to happen
edit: ALSO villagers do not lose their jobs when the job block is destroyed
edit2: villagers that havent been traded with change their trades, no matter if another villger has also taken that job block
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They can change their trades if you leave them too long, also, they can only lose their job during a certain time of day, so just bring some emeralds and paper to lock in their trade for when you get a good book like mending