Suddenly a problem occurred on my s9+ phone as well. Where it should be full black is grey. There is a grayish color when the brightness of the phone is low. It gets better when the brightness increases. I don't know if it's software or screen. Can anyone help?
This is happening to many S9 and S9+ owners since the last updates. I think is intentionally done to force everybody into the newer models... The quickfix is to press the off buton to darken the screen then, with the screen completely off, tap the screen twice to lit it up and bring up the clock. Then press the round button on the bottom center of the screen and open the screen with fingerprint or pin number. It is annoying ! Only a firmware update works. It is also better to turn off the automatic light sensor for the screen brightness.