Well, I spoke too soon earlier.
This case is NOT closed!!!
Yesterday I went for a 22 mile ride with the heated grips set to high for over half the ride and then I turned them down to low for the rest of the ride. I was also trying out my new Kemimoto handlebar muffs along with the heated grips. Everything worked great on that ride. The ambient temps ranged from 7F to 10F so plenty cold for any issue with the controller to show up.
But me thinking that I need to replicate the exact situation when they failed before so I went for a 7 mile ride today. The ambient air temp was around 14F, still colder than when the problem originally showed up. But this time I did not have the muffs on the bars and wore my thickest gloves. The heated grips set on high failed within 2 miles. I did have my phone plugged into the USB port and that is on the same fuse circuit as the heated grips. So I stopped after 4 miles and unplugged my phone to take it out of the circuit. Again, the heated grips set on high failed within the next 2 miles.
This time I paid close attention to the fail flash code showing up on the grip LED. Both times it failed the code was 2 quick flashes followed by a pause, then repeat over and over until the bike was turned off. Per the service manual this indicates that the handlebar heater temp sensor is faulty. Not the controller unit that was just replaced.
It is possible that both items were faulty before but that the temp sensor didn't fail for the dealer but the controller unit did. The controller unit failed during bench testing so it was definitely bad. The heated grips likely worked flawlessly yesterday because I had the handlebar muffs covering the handlebars... protecting the handlebar heater temp sensor from the ambient temp while riding. I suppose if I have to I can just continue to ride with the bar muffs installed. BUT this is all still under warranty and should be working without me doing anything to make up for a failing part. And that is exactly why I went out today without the muffs. I need the heated grips to be working as intended from the factory at any temp that is commonly experienced in my area. And I ride at all of those temps no matter how low they go. OK, I might not ride if the temp drops to -20F (-28.9C). But that temp very rare around here. It is common to see a temp as low as -4F (-20C) around here during the cold season. And I don't want anything to prevent me from riding then other than snow/ice on the pavement.
So it turns out that both the controller and the handlebar heater temp sensor were defective. Rather bummed now since that means another trip to the dealer. And, looking at the weather reports for the next 2 weeks, it is unlikely that I'll be seeing the temps drop again to a level cold enough to verify any repair until next year.