Well, as a test I would "drop " the stairs a bit, to be sure to close the gap that is evident and clearly visilbe, and redo a test render.
(or move the backdrop up  a  bit, same thing)

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"!