If I look very close, and please correct me if wrong.

The front of the stairs is on the ground. Correct?
But the back is OFF the ground.
There is an opening between the stairs and the ground....
I can see light below the rear left bottom corner of the stairs.
I have seen this happening before if walls and floor where not exactly aligned. (gap closed)
Indeed, gluing or welding them together would get rid of this "opening", and "touch wood", the artefacts would, should, could, (be careful here) go away.

check and try it please.

 

Poser 1, 2, 3, 4, 5, 7, P8 and PPro2010, P9 and PP2012, P10 and PP2014 Game Dev
"Do not drive faster then your angel can fly"!