I'm just back from the range and I'm pretty sure I figured it out with some careful troubleshooting. I didn't notice that there was a "sub-button" that is at the 12 O'clock position on the mag release button. It's easy to miss as it's tiny and flush with the frame and almost looks like part of the release button (but it operates independently). There was no cutout for that tiny button on the Nils grip so it was not allowing it to protrude the tiny bit it needs to when a magazine is inserted. I could see on the grip where it had made a mark. I carved out a tiny cavity which allowed the button to move (protrude), and that instantly fixed the problem. 300 rounds and not a hiccup!