How long does the lean AFR last? Does it persist for more than 1 second while accelerating? How are you controlling the vacuum level the engine encounters? Are you loading the engine with the brakes or up a long hill? Lastly, do you know if the secondaries are starting to open during your acceleration runs?
The AP should only cover a short duration lean period immediately following the throttle opening, unless it dumps too much fuel, which will then require the motor some time to recover from. It is possible that the total volume from the pump is correct, but the timing is not optimal. This could be particularly true on the secondary side if those throttle plates are starting to open, since you did a bunch of work to limit the accelerator pump volume coming on that side.
I might be influenced by your answers above, but the other thought I have is that the lean spot is actually occurring before the PV opens. The PV might not be opening at the right point, and the wideband also has a bit of latency in it, so the AFR will be a lagging indicator.
