Want to add, if the bolts were / are titanium, that could have shown similar corrosion that would have started at the counterbore for the bolt head.
Similar to carbon / aluminum, titanium and aluminum together are just as bad if unprotected.
My suspect for the stem to post joint as the origin is based on the white powder and the jagged edge of the broken tab. It still could have been the bolt hole, regardless it is in my thoughts a corrosion stress or crevice type failure.
PK