ya know.. for some reason my brain tells me this could be due to a loose belt.
I know it's hard to adjust the belt. You have to pull that rear wheel back hard and tighten a few of the bolts at the same time.
Under a large load (like when it hits the hill) the belt pulls harder and there's more belt-pulley friction. Under a load, the front and rear variator units act properly.. the rear spring and front weights find the proper resistance and stress is balanced.. So the bike can go faster. That rear spring won't allow the rear pulley sides to open (raising gear ratio) unless the belt pulls hard.
If the belt is loose or slipping the variator just acts srange.
So, also check for a slipping belt.. a bit of grease or oil on the pulleys or the belt can do it.