Downshifting to 2nd at 50 mph would be turning some RPMs, ie; 5000 or more. And the systems threshold is somewhere in there, which means it won't do it. Just turning the Tow/Haul off, which drops it from 4th to 3rd, makes it jump to 3500 RPMs or so.
Our Pacifica has the auto stick option & frankly it's stupid. I've used it trying to be "sporty" & the bottom line is your still driving a station wagon & there isn't anything sporty about it. It does some good when towing though, especially thru long mountain passes. We pulled a boat from Minnesota to Washington last summer & used it a lot.