level 3 librarian switched professions to a level 3 farmer

I had a level 3 librarian with an Unbreaking enchanted book trade which I put a name tag on to keep track of it. It seemed to have claimed a composter which had been unclaimed (I had 3 composters and only 2 farmers) despite not being a novice anymore.  All of its trades are now farmer trades from levels 1-3. It still retained the name tag as well.