Send your level (the FPM file you save) to mike@thegamecreators.com to discover if it is a bug or a strange level design. There are segment called holes that are shaped like windows but have no frame or glass. It looks like you have used these. If you add some grass outside the windows, do the frames return? If so, it is probably a bug in one sense, but would you have a window looking out into the edge of the universe (and into the void of the skybox)?
"We are the knights who say...eki eki eki fatang loopzoing, zanziga....ni"