Villagers will claim a nearby worksite, even if they cannot pathfind to it to claim their profession, thus preventing other villagers from claiming the site. This occurs even if the villager has a workstation that

Villagers who have a profession, but no worksite, can claim worksites for other professions, as well as worksites that they are unable to reach

Reproduce:
-Trap two villagers in separate cells that are close to one another
-Place a work station in one of the cells
-The bug may take a few attempts to reproduce, in the case that the correct villager is able to claim the worksite first

In the attached screenshot, the villager on the right is attempting to claim the composter on the left, despite the fact that there is one that it can pathfind to