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[DISCLAIMER]I'm not certain about this, so it is just a theory for you to investigate further[/DISCLAIMER].....
Its possible that vehicle has a valve to shut off coolant flow to the heater core when Max is selected. If that is indeed the case, its entirely possible the blend door is allowing airflow across the heater core all the time, but the core is cool when max is selected. You might be able to test this theory by switching to normal a/c with the vehicle at operating temperature. When the air is good and hot, switch back to max. If it takes a minute or two for the air to cool off, the theory stands. If it cools immediately, back to the drawing board.