The car has no way to directly run off the wall power, and its limited by the capacity of the circuit its connected to. Preheating consumes around 12-13kW if its warming the battery as well as the cabin with the dual motor, so even if you have a 48A/240V wall connector the car can barely maintain its current state of charge until the heating power drops as the cabin warms. The chargers output just goes to the battery, and the car has no active intelligence or way to independently run off the wall power. It could throttle the preheat power to match incoming power, but Tesla appears to have skipped this method since it would lead to very slow preheating in some cases. So this just means for lower power charger connections you will consume energy off the battery while preheating.
Whats your charger power?
12-13kW? Source? Never heard it being that high.
It is chilly in Seattle right now. I assume you have 40 or 48A charging...if you do....Even without engaging battery heaters, leave your car outside, let it chill, plug it in, set charge limit below current charge level, set heat to max, turn on AC, and leave a door open. You should see that alone reach or exceed 40A. 9.6kW. And then as mentioned battery heating from the motors can go on top of that. Could easily see it drawing down the battery briefly (obviously if not set to max the heat use will back off a fair amount as the cabin warms) even with 11.5kW charging.
I have 48A charging. Never seen my battery drop while pre-conditioning in my garage.
unless the car was actively charging or being driven, maybe that has changed or maybe it should to protect the battery?