This is pointing more and more to either a radiator or water pump issue. If its the same radiator from the smoker that had no issues than the water pump just migjt have an issue with not moving water properly.
since UP has 3 other ST1100s for comparison, for $99 you could compare the radiator thermal patterns of the good ones vs. the bad one and see if that provides any clues.
Powerful thermal imaging cameras designed for your smartphone.
www.thermal.com
I'm not sure how much you'd need to spend on a known good replacement radiator, so maybe $99 isn't worth it to do a little troubleshooting vs. just replace the rad and see what happens.
When I had my infamous water pump corrosion where all the impeller blades were gone and it was just a set of arms with no blades, my temp gauge would read normal at speed. So, your water pump would have to be in pretty bad shape to get the symptoms you're seeing, but every failure is unique so I'm not gonna rule out the water pump either.