Screen Mirroring is ALWAYS off unless you turn it on. And guess how you turn it on? By going into the "Screen Mirroring" setting (I know, Sammy does make some stupid UI choices sometimes...OK, a lot...). If you notice, that's about the only setting that doesn't have a switch on the main page but does once you go into it. So, if you go into Screen Mirroring, it will automatically turn itself on. If you flip the switch and turn it off, it stays that way, but when you go in again to 'check', it turns itself back on. But don't worry, it's off if you flipped the switch.