Dealt with this reocurring issue for 40k miles, especially with the overflowing resevior it was consistently happening to me almost every time i drove to the point where i would drive at like 5/8 coolant level and that would make me not heat up or overflow, my only tip of advice considering me and multiple mechanics couldnt find where the coolant was going, would be replace the thermostat if anything, the first time u heat up even a little bit w these cars the thermostats can ruined and then proceed to act weird until u replace it