I would not use it. Raise hell with who ever sold it to you. Have to be very carefull due to lifetime liability with all work that you do. So many ambulance chasing scum bag lawyers out there who would get pleasure suing their own mother.Just checked Southwire's website and their SimPull NM-B specs. All conductors are copper. You say that this isn't a new occurrence, you noticed it in the past. I would think that if that were the case there would be something from Southwire about it.
Makes me wonder if you got some counterfeit stuff. Where did you purchase it?