I dont understand it either.
Because that used to work since I first bought the f18. I could use the dispense button (without aft or fwd) to dump both flare and chaff in one press, on any prog i set.
Now no matter program its on, it will dump flares and chaff until empty. What is the point to having the dispense button if it can't really be used? If it was used for emergency jettison of all the countermeasure it will dump all 30 flares but have some chaff still left over.