The map options under fog and distance fog can no longer be adjusted by 0.10 and only goes up whole digits or even go into the negatives. This might not sound to big of a deal but it’s literally impossible to make a map pitch dark. (I’m absolutely sure this is a glitch as I have an older map with fine tuned settings)
Might be better to try the Halo 5: Forge support forum for this. Also, have you tried altering the fx to see if that helps?