Hey Dermot,
I've erased the polyline I used to create an opening in the floor, and instead drew a Room Divider wall. Setting that room as Open Below has fixed the visibility issues, well kind of. I see the stairs below, but the graphics aren't showing properly. The stairs are set to show Normal on the floor above, but they are showing as dashed lines.
Also, I foresee the invisible wall conflicting with the railing that will be needed along the floor edge. Is this the only way I'm able to create a proper floor opening for a staircase? Or is there an easier way that I'm missing?