If I remember right, this is happening because you’re standing in a visleaf that can see into the visleaf in that dark grey room. Because there’s a skybox in the way, you can see into the second room- otherwise, it’d still be getting rendered, you just wouldn’t be able to tell.
I don’t remember exactly how you fix it, but it involves either using some kind of optimization technique (hint brushes, occluders, areaportals) to separate the two areas, or redoing your layout in a way that makes more sense. If you’re trying to make a realistic map, just make the room into an outdoor building, or something.
Don’t draw a big skybox around your map; it’s a terrible idea for a variety of reasons. What exactly are you trying to accomplish with this map? That has some bearing on how you go about fixing it.