Hello, I have a Civic LX 2012, my A/C system, Blower, and Blender Motor works perfect, all modes of Vents work perfect, the issue starts when I turn off the AC and starts turning the wheel to increase the temperature of the Car, the front vents starts reducing the air and it starts blowing on defrost, once I reach max, it only blows thru defrost and no matter what I do or press on the mode of vent, is not working, my concern is why it works perfect on AC COLD, but not on the heater?

The air thru the vents selected is reduced as the heater temp is increased, once reached max, it’s stationary on defrost. Why??