I get it. The check valve should work but the point is at high loads the crankcase pressure should be above atmospheric pressure, not by a lot but still... the snout should be below atmospheric pressure since the air has passed through the air filter and throttle blade restrictions. The pressure differential makes the air still flow from the crankcase to the snout.
You may argue that perhaps the bypass is open and bleeding boost back into the snout. Yes, that can certainly happen but in that case your ECU already is bleeding boost. Anything that might flow into the crankcase in this instance will come right back into the clean air duct through the makeup air circuit.
The point is the valve should stop reverse flow but even if it has a marginal leak I don't see a performance loss.