There could very well be a sketchy part in the gun somewhere. It would probably be worthwhile to have S&W look it over to be sure.

The manufacturer's instruction for takedown of the M&P models say to use the internal lever to disengage the sear during disassembly. Pulling the trigger also achieves that, but it runs contrary to the designer's instructions. While I don't know the full reason the pistol was designed this way it's always better to strip the pistol in the recommended way. It's not a Glock.