https://wccftech.com/samsung-discreetly-fixes-galaxy-s9-black-crush-issue-with-may-update/

Like most flagships, the Samsung Galaxy S9 and S9+ launched with its fair share of bugs, the most annoying ones being call quality issues and dead zones in the touchscreen panel. It took Samsung several updates to fix the call quality issue and that one’s settled, for now. Additionally, Snapdragon-based variants of the flagships were found to have a bug which could trigger a device reboot, but that seems to be more of a problem with the Adreno 640 GPU, thank the devices themselves.

One of the less talked about bugs was with the super AMOLED panel being unable to display deep blacks properly in HDR mode, otherwise known as black crush and colour banding issues. It results in a grainy or a pixelated picture with blocks of black which is immediately apparent when watching dark images or videos. The problem is not entirely new as has been a recurring theme across several of the S9’s predecessors.