I assume you read all the above posts and now understand this is normal and not unique to the M&P.
There are two other threads running currently that deal with this.
As a non-retired trainer and "veteran" of over a dozen schools, I can tell you that most any handgun will auto-forward if you insert the magazine just right. I've had it happen on Berettas, SIGs, HKs, and Glocks (where it is quite common). It's never happened on my M&P but plenty of other people report it. It's a combination of the strength of the slide stop spring, the angle at which the magazine is inserted, and the force with which the magazine is inserted.
Can Smith tweak your slide stop spring so it's less likely? Sure. I bet that is exactly what they do for people who complain about it.
But it's not dangerous by any stretch of the imagination and it's a very common, well-known phenomenon that has been around long before there was a M&P semiauto in existence.
More importantly, the specific thing you're describing is even more likely to cause it. You're not getting auto-forward when inserting the magazine. You're getting it when you slam the floorplate after the mag has already seated. If you understand how the slide catch works, it's not surprising this is happening. Again, can Smith tweak your gun so it doesn't happen (or at least doesn't happen as regularly)? Sure. But the sky isn't falling here.