villagers keep their profession even though the job block has been broken

so i brought a new villager into my trading hall. the only workstations placed were composters for farmers. I placed down a lectern and the villager took the profession of the librarian. I broke the lectern about 5 seconds later. about 20 minutes have gone by and he is still a librarian even though there are no lecterns placed in the world with about 500 blocks of my base. He did not take the profession of a librarian until after i placed the lectern.  i did not trade with the librarian at all. this has happened multiple times.