I don't own a Pie but I have noticed some delays for a few things in the soft start BLDC controllers I have owned.
I cant think of any reason why other than Its probably programmed into the software and or hardware to avoid current surges over the fets and onto the
BMS/batteries.
A lot happens when a controller come into regen.
Acceleration needs to be fully shut down, a reverse phase at the right duty cycle needs to be introduced and feed into the motor.
Its possibly related to soft start. There seems to be a delay on brake switches too.