If someone logs into your steam other than you, it resets if they launch arma. If it keeps happening create a new arma 3 Character, change settings, log-out once they're saved, close and re-open arma. Should be the same.
If not then it might be your temporary game files not saving properly, but then you'd have issues with other arma 3 things so doubt that.
NP, only problems i know that cause this are problems with the character file you've saved, so make a new one. Or you're config files being fucked. In which case they may be set to " read only " ( problem with arma 2 ) or in the wrong place. ^^