Have an issue with black crush (gradient) for the s9+.
You can see it in low light conditions when watching videos and even with the blue infinity wallpaper in the top right of the screen.
It's basically blocky detail in dark areas. Lots of posts about this on Android Central and Reddit.
Really noticeable when you spot it. Hopefully a software issue and not hardware.
Mod edit: Thanks for posting this Green_Samot. The other thread we have about black crush has picked up further discussion, so I'll close this one for you guys and instead direct folks to post in that other one here. -AntS