My truck does the exact same thing. With both hoses connected its stuck on the AC vents, if I disconnect the rear one and plug it then it will switch to heat/defrost. When I reconnect it and switch to AC it moves back and stays there. In the winter I leave it disconnected for heat and reconnect it in the summer for AC until I can find a good used actuator.

Like Robert said pull one and plug it and see if anything changes.