Well its so easy that its a shame but what fixed my Problem on a none HDR Screen with the Picture getting always too dark was
simple changing the Nvidia Color Settings to Manual, as let Nvidia decide, not the Program, simple as that I had only push the Gamma
to about 1.10 - 1.20 and the Darknes or Blair Witch Forza Effect was gone for good, not perfect but waaay better then before. Seems that
Forza has a LOT of unsolved Issues with Nvidia Control. Hope that helps some of you, it sure helped me 
I think the game’s lighting model is a little buggy as well. So your setup isn’t entirely to blame.