I noticed that unemployed villagers will claim an already-claimed job site (e.g. I only have one cartography table in the village, but two cartographers). The original villager (the first one that claimed a station) will also refuse to restock despite having access to the block and an available bed.

Edit: Change my example using farmers to cartographers for clarity. 

Update: I tried removing and replacing the job blocks, and the original villagers have reclaimed their proper stations. However, the later villagers (novice level) retain their prior professions and don't claim any extra (unclaimed) blocks I place.