Heat is leaking through vents when off

On all cars I have drove (including 2 other Subies I owned), unless air on the recirc, driving rams air through. not a lot, but sufficient for fresh air in the car

so even though fan is off, air would still go through.

Yep.

Folks, when you set a warmer than ambient temperature, the heat exchanger valve adjusts to let hot coolant flow. With the vents open, it means some forced air comes in, and then enters the cabin. Since it’s a lower air flow than the fan creates, the air is warmer than you may usually expect.

Fan on or off, turn the temperature to what you want, or as suggested earlier, to “LO” to do nothing and close the heat exchange valve.

There’s literally a couple radiators (heat exchangers) IN your car. If the valve is open, the engine coolant flows through them. If it’s closed because the system is set to “LO”, then that radiator isn’t getting heated up.

Here, you can see one of the radiators. The valves are in the engine compartment, IIRC.