Well, it doesnt seem like a HDR issue because it stays dim when moving or looking. It never adjusts back. Also, it gets worse with ENB series but sill exists without it.
I have had this problem before. It had to do with having HDR and water displacement on, with any mods that change water, and or running an ENB. I don't remember exactly how to fix it because different things can cause it. Took me several hours of reading forums and changing various .ini settings.
Set to game mode, which ruined my dark blacks. I had to turn down my contrast from 100 to 50, Increase my brightness a few notches, Turn down my sharpness from 100 to 50.
Poof the dimming is gone, and my blacks are unacceptable.