It appears that the High Dynamic Range (HDR) implementation in Red Dead Redemption 2 may not be true HDR.
According to Eurogamer, the video output of all versions of the game is derived from a Standard Dynamic Range (SDR) image, and therefore is not rendered in high dynamic range. Generally speaking, an SDR image doesn’t offer nearly as much contrast, color range or luminance as an HDR image, and Eurogamer claims that all of the limitations present in SDR are simply packaged in what the game is presenting as HDR.
Ultimately, Eurogamer says using HDR at recommended settings produces an image similar to what you get with HDR turned off, or it can even make the game look incredibly dim or overly bright if settings are tweaked in either direction.
Source: IGN.com Red Dead Redemption 2 Players Reporting HDR Issues