I have never had a issue with loose primer pockets from using brass that was tumbled in stainless steel media. I have 5.56 LC brass that has been reloaded no less then 4 times and each time was cleaned using SS media.
I have 9mm brass that has been reloaded at least 15x and each time cleaned in SS media.
Seeing how he used some new brass and some once fired and both had issues I would point at the primers first.
NRA Life Member, SAF Member