Both my M&P40 & M&P40c did this. There are several threads on some of the M&P forums about it. If having to hold it down bothers you, there is also a "fix". I did the "fix" on both of my pistols and it cured the lever from popping back up.
Here's a pic of the "bump" or some call it a "sprue" mark I filed down. (note: if you screw up the lever they only cost about $5 to replace)