Not sure exactly what is wrong, I would however fix it by making solid mesh objects instead of flipped ones, like this:

Simply to avoid these kind of problems, so if you need a room always model it like its a wall with thickness or make sure that if its a plane, that the normal is always facing the camera. Whether its Poser not being able to handle flipped normals correctly or something, it at least to me is far less problems and faster to simply model the wall in form of a box rather than a flipped one and trying to figure out why it doesn't render correctly :D.