Buzzcut1,
The bracket didn't fail, the bolts holding it did!
As rusty as those bolts are, it's no wonder they failed and likely the heads popped off due to deep rust penetration in the inside head to shank sharp corner. Hope you'd spray painted liberally from multiple angles the new bolts with a rust preventative paint or you are likely to see them break off again due to deep rusting. Rusting weakens steel!
Too bad you couldn't use the farther apart mounting holes also because it gives the bracket a wider spaced bolt pattern footprint which lessens the twisting force on the bottom bolts that are trying to be ripped apart by leverage from the bag's top. Just a suggestion from a retired automotive engineer.