Depending on what kind of staircases they are, it sounds more like the engine problem rather than a bug. You can't stack walkmeshes on top of one another. You cannot both pass under a bridge, and walk over it - in this case you can be above or below... or both above and below in non-crossing parts (for example a walkable balcony overhanging a no-walk floor), but not two places on top of each other that you can stand on.
Things like this have different workarounds - you can place a small trans on either side of a walkable so you 'walk through, ' or make separate sections in an area devoted to top and bottom, etc. Good for small things, doesn't really have the right effect for, say, a tree top village with many walkable bridges and a walkable forest floor...
(Too slow
)
