Sounds like a Glock 19....
It is probably a combination of a new stiff recoil spring and slightly lighter than the gun was designed around 9mm loads.
The Gen 4 9mm had/has the same problem. They design around Euro (CIP)9mm rather than American (SAAMI) pressure curves.
Locking it back so it took a "set" would be what I would have done.
There isn't anything weird looking on the extractor (chipped, moves freely) or ejector (broken, bent funny, mushed) is there?
Try shooting without the magazine and see where the round goes. It might also be that your magazine springs are really strong and pushing the round out of the extractors grip before the slide makes it back to the ejector and it is more noticable on lower pressure 9mm rounds that don't have the same slide velocity.
How far back is the ejector from the breechface?
When I compared a Glock to a M&P 9c and a Sig P226 the Glock ejector sits noticably further back so the ejector hits much earlier in the extraction process on the M&P & Sig which gives less time for weird crap to happen.
I would leave my mags fully loaded for a while too so the mag springs take a "set" and see if that helps.
Last thing, you could buy a new recoil spring from HKparts.net and see if that helps. There will be some variation in spring production that my vary the spring rate some, you might have gotten one on the heavier end of the tolerance band.
(It isn't limp wristing, you have too much experience for that.)