The stairs leading from the lower part of the Noble's plateau to the upper level are very steep. They are fully navmeshed but I have noticed a stutter sometimes when walking up them - and some clipping with your feet through the steps. If you pause briefly your character will rise slightly to be on top of the mesh and you can continue without issues.
I know it may look a little weird ... but it works fine and allows NPCs to ascend and descend normally.