Most cars have a cooler/heater for the transmission fluid in the radiator so even if the temperature is read by a sensor in the transmission, that temperature is dependent on the engine temperature. Without knowing the program GM uses, I would assume that engine temperature is also factored into the program that locks up the torque converter.

 

Next time set your DIC to the Car Temp setting! and drive it when it, as temp rises . see what temp it is when it hit OD,, let it cool and do it again see if it works at around the same temp!! That may help the gear heads help you better! Please post what you find

Just my2 cents