using port A or port B on the KPP should yield same result in this situation.
Are you sure the MC6 is sending only one single PC at footpress? this can simply be verified by connecting the MC6 to a PC/Mac in stead of the KPP (using the same cable: USB-A to USB-c)
how do you observe? looking only at the frontpanel LEDs? or actually listening to the audible result? (on the Kemper audio always has priority!)