Another speculation: we know that Tesla has "overclocked" their new Schuler press by about +16%:
A Look Inside Tesla's Fremont Automotive Factory — #CleanTechnica Field Trip | CleanTechnica
"Matched sets of dies are used in the Schuler to stamp out uniform aluminum body parts even faster than the rate claimed by the manufacturer, thanks to numerous improvements made by Tesla after commissioning the press."
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Changes like this and more allowed Tesla to increase the rate of the press from 12 strokes per minute to 14 strokes per minute, a 16% improvement."
(If this press was sized for 5k/week then I presume the original plan was to buy a second one to get to 10k/week at Fremont. Instead of that they improved the press rate by 16%: which is about 6k/week. Again: pure speculation.)
Running the press out of spec might have invalidated the assumptions behind the original tooling and might have caused that particular (sporadic?) stamping quality problem.
No vendor would accept out of spec use as a warranty case.