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