Are you using the adaptive cruise control? The adaptive cruise will use the brakes going downhill to maintain speed.
If you're not using adaptive cruise, the Flex uses the brake pressure sensor to figure out if you're going downhill or not. It will figure out how much you are pushing the brakes and compare that to the rate of acceleration and from that it can figure out how steep the hill is. It will then use that information to downshift the transmission to give you the appropriate engine braking.
So my recomendation is once you start going down hill, give the brakes a light squeeze so that it can learn what's going on. That should trigger an appropriate downshift and then use the engine braking until you get to the bottom.
If you're not using adaptive cruise, the Flex uses the brake pressure sensor to figure out if you're going downhill or not. It will figure out how much you are pushing the brakes and compare that to the rate of acceleration and from that it can figure out how steep the hill is. It will then use that information to downshift the transmission to give you the appropriate engine braking.
So my recomendation is once you start going down hill, give the brakes a light squeeze so that it can learn what's going on. That should trigger an appropriate downshift and then use the engine braking until you get to the bottom.