On my F150 I find the downshift on hills in tow/haul to be a bit over eager too. It seems to think that you want more engine braking any time you use the brakes, even when slowing for a corner. I've not had it drop to first like you explain, but it had shifted when I didn't want it too. For example, say I'm going down hill and have used the brakes, causing it to downshift. Say its holding 45mph in third gear down the hill just fine. Then a corner comes up that requires me to slow down to maybe 35 or 40 mph. The transmission will probably think that you want more engine braking because you used the brakes and shift to second, which really has the engine screaming. After the corner, if you apply the gas for a few seconds it should gradually shift back up, but its not instantaneous.
Small gripe. It hasn't really been an issue for me in 3,000 towing miles but can be slightly annoying on occasion. I guess if it really bothered someone they could use the manual gear selection function instead.