I was trying to get a villager to get a mending trade on the first level. I was placing and breaking a lectern to try and get it. I had already had 2 lecterns with 2 trades I was planning on making. While I was placing and breaking the lectern for a villager, When I broke it, he immediately claimed one of the other lecterns from one of the other librarians (there were no other workstation blocks placed in the entire village). Upon breaking this other lectern, it had gotten rid of the villager that had the trade I wanted and kept the new villagers profession. So the villager that I was planning on trading with was unemployed while the new villager claimed the already claimed workstation.