The difference to me with the exposition problem, before and after SU5, is that the game seems to be "tuned” for Xbox + HDR TV display. Not only many PC + monitor users don’t have HDR capable display devices, but the monitor and the TV might not be using the same gamma either. In addition, PC+monitors are calibrated for sRGB whereas HDR TV are calibrated for a wider color gamut.
Some good starting points:
“While the Rec. 2020 transfer function can be used for encoding, it is expected that most productions will use a reference monitor that has an appearance of using equivalent of gamma 2.4 transfer function”
Rec. 2020 - Wikipedia
“SDR for HD video uses a system chromaticity (chromaticity of color primaries and white point) specified in Rec. 709 (same as sRGB). […] HDR is commonly associated to a Wide Color Gamut (a system chromaticity wider than BT.709)”
HDR Chromacity
I haven’t done the maths, but your experience in lowering Gamma level to 0.7 seems to reflect the inverse of the change from 2.2 (sRGB) to 2.4 (HDTV).
[edit] given the latest update on SU5 hotfix2 I’ve changed to a conditional tense.