The short time I carried my M&P, I didn't have this problem, but it probably is the holster just because it's leather...leather flexes. I had this same problem with my Lasermax on my Glock...I got up from lying down on the couch and my pants were glowing. I had Rob at Bowers cut down the left side of the takedown lever so I wouldn't push it in the holster. I wouldn't recommend shortening the mag release, though.
Does the leather cover the mag release?