Hydrogen embrittlement comes to mind, it’s a term I would hear re cadmium plating Huey tail rotor drive shaft bearing hangers, I think I heard it once re Brasso and shining up cases.
With nickel plated brass I can see it, not sure how it might affect plain brass.
Probably a batch defect in the mfg process.
I also use the 9mm S&B brass cases for reloads, some have run 4 or 5 subsonic loads and still going strong.
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