The HT-12D has a latched output so the circuit is, unfortunately, functioning as designed. To change the function to match your goal, perhaps we can add some additional circuitry to the transmit side. For example, it would be possible to add a power on/off circuit to the transmitter so that when the power is switched off, the transmitter sends one last message but with the data word forced to be "1111". In order to do this, we would need to add a local power storage filter (for example a series diode following by a larger value capacitor), plus a circuit to sense when power has been removed from the tx which signals the data bits to become all ones, and a gate or transistor at each data input controlled by this sensing circuit.