I had close to the same issue with going down some mountains in Montana. I ran into a problem where I had it on cruise, didn't notice a significant hill and the truck downshifted to 5th going up. Then it didn't get a chance to upshift to 6th before it started going down the other side. In fact cruise was still on and the truck shifted to 4th to keep me at 65mph or so. Annoying, but my own fault.
Did learn you have to hold the gas down a bit to get it to upshift. But that can get you in trouble going down a hill.
Better to switch to manual, and upshift it yourself. When you shift to manual, it will be in whatever gear it was when you shifted to it, so you can easily upshift.
BTW, glad your enjoying your new Ford. I know I loving mine.