Why does Galaxy S9's wallpaper look stained in the dark?

The phenomenon may be due to the usage of default infinity wallpaper with purple-colored background. When setting the infinity wallpaper from lock to home screen, wallpaper is magnified and simple home screen is provided.

When a high resolution image is enlarged, some area may appear distorted and this is a normal phenomenon.

Choosing a default infinity wallpaper with purple background

Top left corner of wallpaper is distorted

Beside, the backround image on the lock screen and power off screen may also appear blurry. This is UX concept and is not related with device malfunction itself.  

Background image of Lock Screen is blurred

Background image of Power-off Screen is blurred

However Samsung will scheduled to release a new Software update for preventing any inconvenience that customers might experience. Therefore, please check for updates to your device SW regularly via the Smart Switch PC or FOTA.