Are you sure it's Generator mode?
Were both banks lit up? Or were you seeing just the left bank lit?
I believe it's true that the inverter is in play when you are driving, but it's just the power being supplied to the outlets in the cabin, similar to the non Powerboost trucks in-cabin inverter.
Technically, it IS on, but not in what Ford calls "Generator mode", which is simply providing power from the inverter to the panel in the bed of the truck. That panel has the 240V outlet, which is what requires the second bank to light up in the Sync display.
Which, by the way, IS available while you are driving if you turn the "generator" on.