Villagers wont lose their profession

I have been trying for quite some time now to get a mending villager in my trading hall. Ive tried to break the lecterns however the villager would not lose his profession but would stay the same even if I haven't locked his trades. This is happening with all my librarians who I haven't traded with. It would be awesome if you guys can fix this bug or tell me what I am doing wrong. Thanks!