Please don't make this a Glock bash but I'd really like to know if your M&P's do the same.
I've been shooting pistols since the early 1980's.
I've had this happen with just about every type of pistol, except for my 1911's and Hi-Powers.
I've experienced it in the following:
S&W 5906/5946's
M&P's
Sig 226's
Beretta 92's
Glock 9mm's
Some designs more than others.
Some internet sites make this out to be a huge deal. I have never found it to be an issue. I have only had it happen while reloading under stress, where I may "slam" a magazine in hard enough to bounce the slide off of the slide release. Even then, It's a rare thing for me.
I've carried an issued M&P9 since the spring of 2006. I've had this happen to my M&P once that I can remember.
When sitting in my home, loading a gun, I'm not under stress. Therefore I don't
slam the magazine, I
insert it.
To me, it's a non-issue.
I'll not bash a Glock 9mm. I carried a Glock 19 off-duty for about 11 years. I still own a few Glock 9mm's. But I now carry a M&P9c when off-duty. When that's a bit too much (rare occasion) my M&P Shield has replaced my PPS and my Kahr PM9 as my official "little gun", and as my BUG at work.
Glockman, if you're interested in the M&P, then try to shoot one. I think you'll like it. Just keep an open mind and don't worry about the whole slide forward issue.