Villagers often seem to link to an already claimed workstation. Also, the villager who has claimed said workstation and the villager who has been newly linked to said workstation both keep their professions. Sometimes removing workstations solves this, but it's not a reliable solution. When inside a trading hall, villagers seem to change previously linked workstations. They do not link to the workstations in front of them, instead they will claim another villager's workstation.