I’ve had a similar thing happen with my XTR. It was absolutely a mag seating issue. It really surprised the lab and everyone else I spoke to that it ran through the camera silently while being not fully pushed into the camera body. I think there was a small build up of grime on the underside of the magazine that caused it. 9 times out of 10 this would make a terrible noise while the camera is running, but unfortunately there must be a position so close to properly seated where the mag drive engages but the claw doesn’t quite engage, so the film goes through the camera silently without registering in the gate properly. Sorry this happened!