Few days ago, out of the blue, my 6s+ (bought in October) also developed a darker grey little shadow in the exact same corner. It's not visible on pure white / black / solid color backgrounds, but I can clearly spot it in the corner of dock bar's grey area if I turn the phone clockwise / horizontally and have the home screen in landscape mode. Probably a backlight issue rather than a software one. I remember seeing something similar at my old 6 last year. Screen quality is not what it used to be anymore unfortunately...