Apparently this problem is more noticeable when you use a pure black image as your lockscreen wallpaper... I've always used black as my wallpaper and lockscreen especially for a phone that uses amoled, as it takes full advantage of the deep blacks as a sort of power saver. However, I've also encountered the grayish filter in the lockscreen but only when I use Fingerprints as my security. The above remedy (change lockscreen security to swipe) works, however, when changing back to Fingerprints the problem still persisted. I guess this can only be solved with a software update and hopefully it will be fixed