I’ve talked to many people with accuracy issues with the 9mm M&P 5” but you’re the first I’ve heard of having problems with the 40.

I believe the word your looking for is obtruation. And yes, you could be getting that effect with your max loads.
The reason your getting keyholeing with your lighter loads (as Tightgroups mentioned) is your bullets are undersized. This is normally due to using an excessive amount of crimp and very common if your using a Lee Factory Crimp Die.
Make sure your expander die is flaring the mouth of the casing only enough to ensure you don’t scrape any bullet material off the side of the bullet while seating. If you expand to much, you lose bullet tension and set back will occur.
Set up your taper crimp die to only remove the flair. Excessive amounts of crimp will score the bullet and swage it undersize for your bore.

As OKFCO5 mentioned in the other thread, plated bullets have given a lot of us problems with accuracy and fouling. Switching to a true jacketed bullet (like Precision Deltas or Montana Gold) not only solves these problems, their cheaper to.

Hope this helps.

Jeff