What does this digital multiplexer (CD54HCT151) output when the selected input is floating?
With CMOS logic parts such as this, any unconnected input may float to any value - High, Low or indeterminant, as the inputs are very high impedance.

ALL inputs to CMOS logic parts MUST be connected to something: the output of another CMOS part, or pulled high or low via a resistor to Vcc or Ground, or tied directly to Vcc or Ground.