Dramas with 5/16 head fasteners

Have not had any strippage. But I have had 2 occasions where one of those 5/16" studs broke off just like yours after many miles of running. One time I was able to remove it with my fingers. The last time it broke off flush with the cylinder barrel. A local engine machine shop was able to drill a perfectly centered hole into it and get it out. (No charge - he said it was a cool bike.) I have only seen black studs. I thought I was being pretty religious about tightening properly. Maybe a bad batch of studs?