If you're referring to the wide, flared part of the frame molding where the locking block pin is in the frame, that's a deliberate design. The "recessed" area of the frame to the rear of the pin is needed for the takedown lever to have clearance for its rotation. The locking block frame pin "bisects" the offset in the frame on the left side.

The left end of the locking block pin ought to protrude from the left side of the frame a little more than on the right side so the end of the pin is level with the wider, forward flared outside of the frame. It acts as a "stop" for the takedown lever when it's rotated down and forward for field-stripping, to help prevent "exuberant" rotation of the lever from letting it hit and mark the frame.

This offset of the outside of the M&P frame at that spot on the left side was initially (mistakenly) thought to be a defect/frame crack when the M&P's were first introduced 10 years ago. It wasn't until the guns were getting to be in common circulation that the mistaken internet rumor was put to rest.

Still, someone new to the M&P pistols could easily think the offset, flared spot of the frame looked weird on the left side.

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Ret LE Firearms inst & armorer