Out of curiosity, what ammunition was involved? Not one of the 'budget brand', lower power 115gr loads, was it? (I mention this since I know of an instance where a major brand's low-cost/budget contract load was once found to have a case rim tolerance issue observed by the factory when they were trying to identify a feeding problem in their previous SW99 that was reported by an agency several years ago. Sometimes it can be an ammunition issue which causes what's at first suspected to be a 'gun problem'.
)
FWIW, I learned in an update M&P armorer class that S&W apparently received a small number of extractors that had been over-molded on the leading edge of the extractor hook, but they identified and corrected that issue. (It was discovered when investigating an issue reported by an agency evaluating some M&P 9's a couple years ago.)
The extractor also received a minor revision within the last year or so, involving a sharper hook and a little more negative rake. The same extractor is still used across the whole model/caliber line.
I've not seen any extraction issues across a number of 9, .40 & .45 models owned and used by different folks, and I wouldn't expect it to be a 'widespread' issue. I'd give them another chance to correct it under their lifetime warranty, especially since the shipping is covered by them for shipping both ways.
Be sure to articulate in your accompanying letter that it's been returned twice for an extractor-related issue (so they can also examine the barrel for any spec issue). Also identify the ammunition that's been involved.
Just my thoughts.