I think I've mentioned this before but whatever...
RA2 reuses textures if they have already been loaded. Therefore, if there are any image files with the same name but different looks than this tends to happen. It happens to arenas and components all the time. To fix it, you would have to change the name of the skin that is being used by the arena's floor in the GMF and in the maps folder (preferably in the other arena since this one came out before it).