so the heat valve is supposed to be plumbed in at a specific way.
I think it has to be on the output side of the heater core instead of what you would expect the inlet.
It has to use the "drag" of the heater core to help stop the flow.
If it is plumbed in wrong, it allows the hot coolant to push past it.
Eventually it overwhelms the a/c and you get warm/ hot air.

The question is of course has it always done this, or is it a recent thing?