I have a Space Grey, 128GB iPhone 6S Plus. Bought it in November 2015 and the first signs of shadowing in the upper left corner appeared 6 months later. I went to Apple Store (in Brazil we have the amazing number of TWO Apple Stores, so you can picture the size of the line we have in support) in October 2016, and the screen was replaced without charging me for it (warranty was still good). Apple support informed me that even though my warranty would expire in 1 month, there was a 3-month warranty for that tech support. So, my phone was perfect for the next 6 months, and the boom. Same problem again, exact same shadow, same corner, same size. I could not believe, how unreliable this screen is!
One more time, went back to Apple Store and for my surprise: they could not replace my screen again, or the phone alltogether, without charging me (Brazilian costs: 900 BRL for screen replacement = ~270 US dollars, or 1600 BRL for phone replacement = 484 US dollars). This is now a 2 year old phone that happened to have the SAME EXACT PROBLEM twice, after 6 months of use each. But Apple does not have a quality program for this iPhone 6S Plus screen, so all of us can pay for several screens, every 6 months.