Fog can act like a zone portal and stop things rendering in the distance. In this case, you have to find a balance between fog distance settings and distance of hallways, etc. The fog is basically chopping off things in the distance and not allowing them to render which is causing the lightness. Increase the distance to fog end so it's a little larger than the actual distance of the passageways. It's also possible to assign a minus value to distance fog start so it's thicker closer.