My Wingmaster will do the same thing, and so would five others, and three BPSs. When the gun fires, the action is unlocked. The bolt is still locked to the barrel. Recoil throws the entire gun backwards, and you stop everything, but the bolt and fore end.
It's a pump gun thing. Perfectly normal. As long as you cannot pull the fore end to the rear when the gun is cocked without depressing the action release you are fine.