From another board:
"I actually don't think the frame being flexible would be a problem, the problem is that the guy in the video is ignorant about mag retention theory in some European circles. FN designs mag to be retained upon release and they expect you to manually remove the mag. I experienced the same issue when I first bought my pistol and talked with military friends and called FN, their intent is to prevent loosing a mag during the fog of war. The FNX was designed for a U.S. conract competition for our military as a replacement for the M9. It makes sense that you would want to retain control over the mag in combat, you aren't shooting timed competition and you will only have thee mags available and getting replacements wouldn't be easy."