I've had that problem, started with a bad stator found after the bike wouldn't start.
After changing the stator and rectifier with aftermarket parts, about 5,000 miles later I kept burning out headlamps.
Found out the rectifier was pumping out 18V!
Changed the rectifier, all was well for about 2 weeks, then intermittant headlights.
Rectifier ok, stator ok, battery charging normally, fuses ok.
Changed the relay package.......then the low didn't work at all.
Changed the starter relay.......everything works.
I need a headlight plug Voltage reading please?
Funny thing is, this morning driving to work..........no low beam (10,000 miles since the starter magnetic switch replacement), I'm hoping its just normal bulb burnout!!!