Dark Shadow on Top left Corner of Srceen IPHONE 6S

I am using a iPhone 6S - 64GB gold, which was bought it November 2015.I am wandering if anyone is experiencing the following concern that I have : Across the top of the screen on white backgrounds there seems to be a dark shadow cast from the edge.The dark shadowing is the corner near to the "Volume/Mute button" . The corner where it display the network coverage signals from the service provider.

I also have done restore to factory default settings and also upgrade to latest 10. IOS , all the solutions given from Apple dint work out and they suggested to screen replacement, seems Apple store in Dubai ( Mall of the Emirates) also dont know what the issue is. Instead they said give us the phone for 24 to 48 hours and we replace the whole screen for you. I dont have any other phone and want to know if this issue will not spread further on my screen? Does any one experience like this which did not spread further and are still using it ? Or if anyone share their screen replacement or repair from Apple and how did it go with your phone? and if the dark shadow was removed?

Thank you