I have used a lot of edge equipped phones and have never seen it, light fall off maybe but not a clear shift. On my BOE equipped M20 pro (now returned) it was clearly visible on any app with a light background, when I ran display tester it was even visible on a yellow screen. Photos of mine attached, actually looked worse in the flesh, hard to capture properly, Note also included for comparison.
I have seen in store display models that do not look quite as bad, wonder if they are non-hulk LG screens.
Just took delivery of the view 20, having a clear out of other phones so am on the fence as to whether to give the Mate 20 one last go....