https://www.reddit.com/r/xboxone/comments/7qo9r4/forza_horizon_3_hdr_too_dark_since_4k_update/
This is a post on Xbox One Reddit with many responses.
Personally I have a Samsung HU8550 With a SEK 3500 EVO KIT. I have had this darkness issue since the beginning, every other game I play with HDR is bright and looks great including Forza 7. My TV is properly calibrated, and adjusting in game brightness and HDR brightness doesnt fix the issue.
Sad to say I was happy to see other people now having this problem as maybe it will finally get some attention.
This guy has a potential reason for the problem, i copy pasted this from reddit:
[–]x3n0n1c 2 points 1 day ago
Not really the case.
The way HDR is mastered is the content is tone mapped to a specific nit value curve, with a maximum nit value for pure white. The HDR10 spec calls for up to 4000 nit highlights but many content creators don’t master to that.
Anything within the capabilities of the set should display at the nit level intended by creator. It’s only what happens outside the capabilities of the display that is up to the TVs internal tone mapping to deal with.
FH3’s mapping is just off, they have set midtone values too low.