I just replaced the recoil spring on my M&P9 FS and I noticed that with a new mag loaded to 17 it will drag the top round in the mag. I did this with a snap cap in the chamber and dummy rounds in the mag. I didn't see it carry over to live fire though.

If it's only happening with the 10rd mag I would say it was a mag problem, not a problem with the 40c itself.