I could be wrong, but this looks to be similar to the unit we use on the MINI. Is the bypass actuator spring loaded?

Ours works in this way: There is a spring that holds the valve shut. Engine vacuum pulls on the diaphragm, countering the spring and pulling the valve open into "bypass" mode.

So when you start the car, the valve is closed proving positive flow though the TB into the engine. Once the engine starts, vacuum pulls the valve open. As the TB is opened more air is allowed into the blower, which is more than the engine can digest, creating a positive pressure allowing the valve to shut.

On the MINI you need a positive vacuum source to open the valve during boost to provide a failsafe in this manner.

Again, there could be something else going on here I'm not seeing.