You are correct in your statement that the ISCV should be closed before the rear turbo comes online. And it is bad that it isn't
My suggestions,
Step 1) Get a multimeter / test bulb and see if the solenoid is actually being activated by the engine ECU / a short along the control wires for the VSV...
Step 2) Check to see that the VSV is working correctly, i.e. it doesn't allow the stored pressure through the valve when OFF.
Cheers
Wilbo
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