I think it´s a nice and useful feature when having dual-sim available. My girlfriends LG G4 H818 with Android 6 has the same feature and mine is actually unfunctional. Yes, I would also like to have it working. Since the option is implemented already, why not fix the bug. I could test it on Android 6 but I am most sure it will work properly. Her phone isn´t rooted, so I´m wondering what for it´s an issue of rooting. I also have tried the option with the privacy setting and moved my ringtones and notifications to internal memory on my FP2, but nothing helped.