North-West is a pile of impassable debris (always there, not random), South-West is just a wall.
Edit: upon closer examination, the North-West corridor is indicated as blocked (there’s a black “gap” in the ground), but it’s really, really hard to see when looking at a distance. It could definitely use some clearer indication. The South-West passageway is still a solid wall though.
And since I’m already adding a different bug to this thread, I also suspect that the Arcane hero Speldrakus is bugged the same way Dar’Vagol the Mageclaw used to be. He uses all of his abilities but never seems to dispell.
Edit 2: Scratch that, Speldrakus does dispell, I guess he was just being inconsistent with his rotation in a few fights.