Your post is very interesting. I just purchased a 2021 Limited Hybrid with the Class III package. With no trailer connection, and using a volt meter, I attempted to confirm that Pin 4 (12V - Charging) on the 7 pin connector is active. With a signal light on, while I was measuring various pin voltages, the tailgate mysteriously opened twice. I have the auto-open feature by swinging your foot under the back of the vehicle, however I was behind the hitch and not swinging feet or hands underneath. Now I wonder if I accidentally came across the same issue you describe???