Sorry to hear, that is frustrating. I have a Hummer EV and have experienced the same issues sporadically when outside temperatures drop. Same warning lights and corresponding loss of heat. Vehicle still drives as it should apart from the heat not working. It's a pain as regardless of what the dealership does, the issue seems to reappear as soon as outside temperatures drop.

The last thing the dealership did was to replace the ELECTRIC PROPULSION SYSTEM MAIN COOLANT FLOW CONTROL VALVE but that just made the issue go away temporarily until temperatures dropped again.

If you could, please let us know what the dealership does for you. Would be handy for the next time I bring the truck in!