Grey shadow on bottom left corner of iPhone 8 Plus

Worth it to try a Forced Restart on the phone. Follow the instructions very carefully, as this is a bit tricky. (Hint.....ignore the "slider" when it appears and keep holding the Side button in until the Apple logo appears. This might take another 30 seconds.)

Force restart iPhone - Apple Support

There are only 2 possible results when you perform a Forced Restart:

1) The phone / screen returns to normal operation

If the shadow on the screen remains.......

2) There is a hardware problem with the phone......the screen is likely failing......which will need attention at the Apple Store (make an appointment) or take the phone to an authorized Apple Service shop.

You can find a list of authorized service shops in your area using the link just below:

Find Locations