I will have to investigate this. With outside ambient temperatures in the 70's I never touch the heating feature. However, I don't know if it's set for using outside air or recirculated air. Would that make a difference? As long as the climate system is set to "off" it shouldn't matter if he air is recirculated or outside.The big hit for EV range is the climate control system. What temperature do you have set for the cabin? Are you using outside air, or recirculated air?
Is the car using the heater to warm up the cabin?