IIRC the M&P has a lever inside the magazine well that you push down, while the slide is locked back, so that you don't have to pull the trigger to field strip the weapon.
That may only be the ones that have the magazine disconnect, but I'd like to hear from M&P owners about whether the trigger has to be pulled.
As to your problem, that does seem weird. Are you sure that you have the lever rotated all the way down until it rotates no farther?
“There’s a rattle in my glove box,
A Federal twelve-gauge slug.” – Adam Carroll in song Porter Wagoner