Modern cars are designed to force air to the defroster if there is an issue with the system. The defroster is the only safety related use for the heater, so they have the system go there. The reason the A/C is working there is because the A/C always comes on with the defroster because it dries out the air and helps cut the fog quicker. I would guess that you have an issue with your heat controller, but without some diagnostics, it is just a guess.
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