If a villager thinks it can reach a workstation even though it can't, (by using trapdoors) the villager will tunnel vision on that workstation and it wont accept any other work station that he can actually reach from where he is standing at that moment.

for example, if the is a workstation 10 blocks away from a villager. the route is blocked by a trapdoor, so the villager thinks he can just walk to it, he will get stuck against the trapdoor thinking he can walkright through. if you would place a new workstation right next to him that he can reach, he is still locked on the workstation behind the trapdoor. the villager does not realise that there is a workstation next to him.