I have a Samsung CHG70 HDR monitor connected with DisplayPort. I typically do not have HDR enabled in the Windows 10 UI because it looks bad and most games that support proper HDR will simply switch the monitor to HDR mode as needed. If HDR is enabled in the Windows 10 UI, games that don't natively support HDR typically switch the monitor to non-HDR mode, however, I've noticed some non-HDR games not switching out of HDR mode.
Detroit: Become Human (Steam demo) - Has an HDR option in its config file (GraphicOptions.JSON -- "HDR": true,), no option in the in-game settings, doesn't switch to non-HDR mode, doesn't look like it's really doing HDR. It does support HDR on PS4, but I have a cheap TV with very basic HDR (DisplayHDR 400 equivalent) so I can't do a comparison.
Team Sonic Racing - No indication it supports HDR, but it leaves the monitor in HDR mode
Both games are running fullscreen, not borderless window. Could it be that they're not in exclusive fullscreen? Is there any benefit to leaving the monitor in HDR mode for these games?