Apple iPhone 7 (Rose Gold) - Faded Colors on Rear Camera

Good day!

I have an Apple iPhone 7 (Rose Gold). My problem is that I noticed recently after I removed my case is that there is a discoloration around the rear camera of my phone, mainly the same shape with my case' hole for the rear camera and flash.

You can see the attached picture for reference. My iPhone's warranty has expired. It is just weird, why this happened?