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.
