Multiplexer CD4067 bug:signal appears in unselected channels
You need to connect the unused channels to some particular voltage, not leave them floating. 

You are violating the 10K\$\Omega\$ maximum source impedance requirement for the microcontroller by leaving them floating so you will get a reading that depends on the previous channel selection and other things. 

Adding a resistor on the output of the multiplexer is not a very good idea, as you have found, because not only does the multiplexer have significant internal resistance (around 500\$\Omega\$ typically) but it varies significantly with input voltage and with temperature. 

So, don't connect anything other than the ADC input to the output of the mux and don't leave the analog inputs floating if you want the ADC to give you a sensible reading. There are no bugs here. 

Because of the 10K maximum source impedance, you probably should not expect to use a piezo sensor directly with the micro, you may need a high impedance buffer amplifier (unless you can parallel the sensor with 10K and still get enough signal to be useful).