I have had that happen a couple of times now, I thought i was losing my mind, but yes finally figured out it was the bike and not me, the pedal was going way down, it has to go to the dealership to be checked, but i noticed how close the reservoir is to the exhaust, and i'm wondering if the brake fluid is over heating, with the combination of heat being generated through the system while braking, combined with the exhaust heat, if the oil gets to hot it will cause the brake pedal to fade, if it gets really hot, beyond the oils heat rating, you will lost all rear braking ability, i was debating making some form of a heat shield to see if that makes any difference
i did notice that if i stay off of the rear brake for a bit when this happens, it comes back to life again