No-your car does the same thing in Defrost.The air passes over the heater core and the evaporator core.The cool evaporator core removes the moist air which dehumidifies the cab,then is warmed by the heater core.Sometimes it's a balancing act between enough heat and defogging as the cool evaporator core reduces the heater output.The fogging is even worse with the blower!Good Luck!