Mine has always sprung back if you didn't hold it. Owner's manual says to rotate the lever down and hold it, so I would think it's not a flaw that it flips back. Then again, in searching for a diagram to see what could be causing it, I found an armorer's manual which just says to rotate it down...nothing explicitly saying to hold it down. Without being able to see inside, it looks like the guide rod/spring is pushing against the take-down lever, which when flipped down should have the round side of the 'cam' against the guide rod, so to me, it should rotate back if not held down. My two cents.
M&P 9, 9 Pro CORE, Steyr S9A1, L9A1, Kahr PM9, Beretta 84F, PK380, Buckmark UDX Plus, 5.5" slabside