Samsung Galaxy S6 — Screenshots are dark with diagonal lines

The screenshots are dark with diagonal lines on it which has been happening for the past few months. The samsung service centre lady suggested it could be because of the screen guard as it could be covering the censors. I've taken it off and it still does the same. I noticed that even when I screen mirror my phone to the TV, I can see those dark lines (same as a saved screenshot). I've tried to clear cache and force stop the gallery (as suggested on one of the forums, worked 2 or 3 times but then back to square one). Hope to get a solution for this.