Its like this.. On a stick when you shift it blows off and then reboosts to get to the desired level. On an auto tranny it builds boost and when it shifts it doesnt blow off if your flooring it.
It shifts to the next gear and slightly drops to 1-2psi and stays there until the next shift when it goes up again.
My guess is that when it drops to a lower RPM after it shifts the fuel pressure changes causing the change in boost. But why it doesnt build back up to the max boost is beyond me.