Got a M&P pistol armorer at work (since you carry a M&P 40)? Who knows what some budding young "internet taught, kitchen-table gunsmith" might have done to that particular M&P 45?
Let an armorer give it a once-over if you have one on staff.
What color are the mag springs in the .45 mags? Orange? Purple? Light green? Any of the coils kinked/bent? The light green painted springs are the latest, last I checked.
I've fired 5-6 different types of duty JHP's through my own M&P 45, as well as a couple brands of ball ammo, all of them feeding & extracting/ejecting with monotonous reliability.
I've used a 3 color mag springs, although one of the original orange painted ones acquired a weird bend/distortion (which didn't interfere with feeding, oddly enough, but it got replaced right away after i discovered it). I'm still running a mix of purple and new light green springs, but the next time I place an order for new springs I'll replace all the purple ones with the latest ones.
Make sure the extractor hook is clean and dry. Ditto the mags.
S&W might offer to have it shipped back to be checked under their warranty (even though you're not the original owner, but they also might not even ask), simply because the M&P is their flagship pistol line.