Hard to guess but could be popoff valve is either not opening at right time, taking too long to open, installed in wrong location or in some cases I have seen where 2 popoffs are needed.

Also could be too much timing in the areas where going into decel, still too much airflow and timing is set assuming there is less airmass or due to time during upshifts while lifting on gas pedal closing butterfly of TB but too much airflow due to boost