its not a bug, its a feature. It happens when walls are thick. Walls behind which there is void doesn't count because you need an opposite face for the game to measure the thickness. In crossfire many buildings actually have inaccessible interiors with thin walls to minimize selfgaussing, modern compile tools automatically remove such inaccessible areas, which can be disabled with a certain compile paramter. Something like -nofill, not sure exactly.
If your floor is a thick brush and your entire map is enclosed in a big skybox textured cube, thats why it happens.