It's the air shaft leaking into the lower leg and pressurising it. You may find that with the rebound closed then the front wheel lifts off the ground when checked.Every 3 rides like clock work I need to let all the air out and then bump the Allen screws at the bottom of the fork And then refill it 20 lbs at a time pushing up and down on the fork to equalize the negative air chamber. It will go from harsh to super plush every time.
I have exactly the same issues with my fox 38s and awaiting warranty work to sort them out.