Looks like a low pressure round that didn't expand far enough or fast enough to make a good seal. Did any gas blow by into your face? If not, I wouldn't worry about it. My low powered reloads do this all the time.
-Deke
I sent my wife's 3913 to LSG Mfg (An authorized S&W service center) and had it back 4 days later with a new extractor. Everything, including shipping both ways, was covered by S&W. You might want to give them a call.
-Deke