Did you find that the steel tongue was a little rough? Mine had traces of copper on it after use, until I polished it up a bit, now it’s fine.
Just checked my back-up mag and the only mark on a round was #10 of 10, a slight scratch. Not enough to shed any scratched brass into the mag that I noticed.
Felt the edge of the tongue and it didn’t feel sharp. Maybe you got a bad one.
If it is a new mag, I read they can be stiff and need “breaking in”. I never thought I’d get the 10th round in until I bought one. Good purchase...