I have had a Mikuni auto choke reactivate from freezing temperatures. Did it to me on highway cruising speeds a few times - it would just lose all power and die. I'd be forced to pull over and let the engine idle for 5 or so minutes before the choke disengages. What I can recommend is a little sketchy but I was out of good options. I used an offcut of core flute (the plastic version of corrugated carboard) and cut a square of the stuff big enough to restrict air flow one one side of the radiator from the front vertically. Abut 1/3 of the surface area should be enough. I drove it to work first up before sunrise and when I got to my destination, I pulled the restrictor out as the afternoon temps weren't cold enough to mess with the choke. Your other option is to take the thermostat out and swap it for a winter rated unit.

Your choke is working perfectly by the way, it's just freaking cold...