I had a base on a small plains island. I've placed workbenches and tried covering all spawns on the entire island. Not even getting spawns from events really. Once a troll attack was triggered and one troll spawned on a nearby island and tried to swim across. Other than that - not a single spawn inside once everything was properly covered.
Are you sure that they spawned "next" to your workbench and not in a non-covered area and then came to the workbench? I've had that a few times, then had to really find those "uncovered" spots and place workbenches there as well.