iPhone 6S - iOS10 - black stain

Since I upgraded to iOS10, I noticed one strange thing. While I hard rebooting (home+power button), in the moment phone shuts down, black, kind of triangular, stain appears in the upper right corner of the screen. I appears only in split second and then after phone starts booting again, disappear. I also put back iOS 9.3.5 just to check if this is also happening, but iOS 9.3.5 is different in a way that when I did hard reboot, it immediately shows black screen (not white like ios 10), and I can not see black stain on a black screen of course.

Now I am worried if this isn't maybe some hardware issue, but I guess if it is, it would appears also somewhere or sometime else, and not only here.

Here's a picture and a video showing this issue.

https://www.youtube.com/watch?v=6KwqM6WlfA0