Do you have a picture of this? I'm wondering if I have the same issue. I see some darkening at the very top of the screen, a few spots that I can best describe as different light and dark areas. It only shows up on white/light colored backgrounds, like for example iCloud Drive. It is very minor, but it is there.
Sorry, I'm using the new phone now and have no issues anymore. Yes, my issue before is very subtle and can only be notice in a white background. It's like a soft shadow on top of the screen (multiple). Make an appointment at Apple Store if you're not satisfied. They will exchange it right away. They told me that my phone still considered like an apple care + because it's still new but after 2 weeks, it will be in a regular warranty so I guess it will be harder to exchange it if the issue is minor. I suggest that you do it now before 2 weeks runs out. We're paying a premium phone and there's no reason not to return it if your not fully satisfied.