For winters, its normal to do so, because it's already cold weather (less AC load), but in summers, doing the same will result in excessive AC load. Many newer cars with climate control frequently use that on auto mode in winters.
But in summers, a little bit mixing of cold and warmer air (hardly noticeable without temperature sensor) is only going to decrease AC cooling performance. I noticed 1-3C degree lower vent temperature (measured with DMM temperature probe) after heater pipe disconnection in multiple old cars (well maintained), because a very tiny bit gaps are there in 20 year old cars, that let warm air from heater core to mix with Ac cold air.
Flaps, that control airflow, basically have foam for air sealing, but by 20 year old, they are semi worn out and do not close 100% and let a tiny amount of warm air leak. I replaced all those foams, but still flaps occasionally remain very slightly open.