Yeah, that's the same experience I have with the one on my Mossberg 590. It seems like light loads/birdshot don't build up enough momentum to overcome the spring tension.
However, once you start getting to buckshot, slugs, or 3" magnums, it feels just like the light birdshot.
The best way to demonstrate this is to take the unloaded gun and tap the butt on the floor. A light tap won't do anything. But a harder tap, and you hear a "SKA-SQUEE".