What am I missing? Looking at Fisherguys PD 9280 chart, I see for the first ~40 minutes the PD is in current limited mode and the voltage is reduced. Between 40 and 50 minutes the PD comes out of current limited mode and may be limited in power. Beyond ~50 minutes the battery does not accept the full 80A and so the current drops while the voltage remains roughly constant. Boost mode ends at 200 minutes.
If I understand correctly, you want the current to stay at 80A for the entire 200 minutes, even though the battery is charged up pretty well.
Seems to me that the PD is doing pretty well except for some power limitation in the 40-50 minute time.