Yeah, the first popup thing before the kovu screen pops up actually changes the rendering system back and forth for me. I'm not allowed to select the other when one is present, only one option shows. I have to go into the .cfg file and manually change it, and even at that it still says LOL NOPE you gonna play dis one. It's erratic.
Yes, most of my maps have custom sky files. But some are also cave maps which have no sky at all.