I've been having the exact same issue as well with raised black levels. The worst game for it for me, is gears 5 on xbox one x. This game is supposed to be reference hdr.
No matter what settings I use, the black levels seem to have a washed out look and intro screens are not inky black such as coalition logo screen etc, unless I turn the brightness down to 49, but this causes alot black crush and splotchiness, which I have to reduce the ingame contrast and increase the brightness to fix and I end up with washed look again.
With my panansonic oled which I had before, black levels were really inky with this game. Now with the c9, it feels like I'm playing on LCD screen with the backlight set quite high. Not sure if it might be the pana oled crushing black levels maybe. As the loading screen and lobby screens in gears 5 are inky black.
But forza horizon 4 and bf5 seem fine.
I've tried switching DTM, still I have raised black levels. Hope lg with release and update to fix this problem, as there seem to be few with it.