I tore my P226 down to get a better look. It appears to be the back of the trigger bar that is engaged by the decocker.
It is hard to tell about the mag without a close inspection, but it is possible I suppose. Mags could be just a bit out of spec, Sig no longer makes their own mags. Turn around time has been a couple weeks, for items I have sent in. Although none have been warranty repair so it may be faster especially if you push them for a rush service.
My bet is that the part itself was faulty, bad heat treatment or something similar. Looks and checks fine, but failed under stress. It happens to everyone once and a while.