After Breeding new villagers they (the unemployed ones) have kicked master-level villagers off of their job blocks. For example, I had a single master-level librarian attached to one lectern. After breeding new villagers, the master-level librarian has disappeared (or, as it would appear, switched to unemployed) and a novice has taken its place with a new set of trades. This has occurred for multiple villagers and multiple job site blocks. During this process none of the job site blocks were broken, and no trades were made with the novices, however the master-level villagers have not yet returned.