Assuming you are using the cabin heater (I know I would be!) that could be why the engine is coming on as the resistance heater has limited capability. In conditions that cold it might need supplemental heat from the engine. One way to test this would be to precondition, then start driving with maximum seat heat but no cabin heat and see if the engine still comes on. If the engine doesn't come on, then turn on cabin heat and see if that causes the engine to start.Here in upstate NY were are in a cold spell with nighttime temps at 0 to -3 below.
I got the car last year in late March and this is the first time I have had the car in the above conditions.
The last 2 days I have found the ICE starting up in EV mode with full charge now down to 28 miles on the meter after just 10 seconds of operation.
I have not seen posts mentioning if this is normal for these temperatures, also wondering if other owners have experienced the same behavior?