I am not trying to be a hater here, but you need to reverse this setup and abandon the idea all together.
#1 - Boost control is done via the wastegate actuator, wastegate solenoid, and ECU. How are you "adjusting" boost via this setup?
#2 - The sole purpose of a BOV is to keep air moving when the throttle plate closes to protect the turbocharger. By adding another, you are decreasing the efficiency of the system.
The airflow path out of the valve must be as short as possible or not exist at all.
#3 - Are you having a boost spike at 21 psi or are you actually holding 21 psi for a period of time? The vacuum line coming off the back of the HKS valve, where is that hooked up to?
Ultimately you are not doing yourself any good, my professional advice is to remove it and replace the stock valve if you want an aftermarket setup.