Not entirely sure if this is a bug - or if it is even desirable to fix it.
I am using "outsidefog" in mapinfo for a map I'm making. During the game, a building gets damaged - its ceiling gets a nice hole in it courtesy of instantly lifting the ceiling and changing the ceiling texture to F_SKY1. When that happens, the sector effectively becomes an outside one, but does not become foggy.
I'm guessing the outsidefog stuff gets set up at map initialisation and cannot be changed afterwards. (Hmmm, I'll have to try changing a ceiling from F_SKY1 to something else - I suspect it will stay foggy). So, anyway, like I said, possibly not a bug, and probably nothing to "fix" but I noticed it so there you go.
If I really want the sector foggy (and I'm not sure that I do) I guess I could script a fog change too.