As mentioned, I suspect recoil + lever inertia are overcoming this tension and the lever drops and the action only appears to "unlock" under chamber pressure. A call to Marlin might seal the deal for you if you still have concerns.

Somewhat off-topic but vaguely related: Ease of lever operation was one feature that assisted John Browning in his development of his first machinegun; a gas-operated Winchester .44-40!!