that is good information to have ...

could the issue been with the bolt itself? a reused bolt that was over torqued at some point in it's life?
with 5 bolts on your mount i find it difficult to accept that one bolt would break off and the others are still fixed. if the other 4 were loose and all the stress was on that one bolt then maybe that could be the case
or, if the bold wasn't broken could the installer have over tightened the bolt and stripped out the threads?
or, could someone have rethreaded the hole and stripped the hole?
and
add to that the gussets that are on your mount would surely resist any twisting in one particular location ... no?
i am no engineer but that is my reasoning behind your broken bolt.
on mine, the bolt holes were all rethreaded (read cleaned)
the mount has the 90 degree bend at the front
the 90 degree reinforcement bend at the bottom edge to the bottom section
the extension across the face for the upper York bar mount.
i can not picture an issue with the upper bolt ... but ... i will take another look and see what can be done to strengthen that particular location.
