A village that I had been working on had an armorer in it. I leave the village for a couple in-game days and when I returned, the armorer was gone and a fisherman had replaced him, even though all the fisherman job sites had been claimed already and the armorer job site was still present.

This meant that there were four fishermen, but only three fisherman job sites in the village.

This issue with villagers changing their former professions by claiming a different job sites that has been already claimed has happened on a couple of other occasions as well