This is the solution. Knox has a thing called SECURE FOLDER. If you uninstal the secure folder app it will fix the problem. It worked for me. You do not have to uninstall Knox.
I'm having this same problem. Started after I got the official marshmallow update. Can't figure out how to turn it off.
Turned stay awake on and off. Both makes the screen turn on at 100%
Tried using the screensaver option too. Also turns the screen on.