Raising ground level of Wemos ADC from Multiplexer
When anything other than address 1 is selected Y1 will attempt to go to +12V. This will exceed the input rating of the 74HC4051 and almost anything can happen.

Most analog multiplexers do not allow any input voltage to go outside the supply rail limits or current can leak through unintended paths within the device.

It will work correctly when Y1 is selected because the voltage is within range because of the load of the resistors in the D1.

It looks like the 12V is leaking to all the other inputs under the fault condition.

If you want good accuracy I would not recommend relying on the resistor values of the D1 Mini. I would instead use a relatively low-value resistor potential divider to bring down the voltage to one acceptable to the D1.

The resistor values in the potential divider would need to be much lower than the values in the D1. Say 10K ohms.

If you just want to avoid the fault you could use a diode clamp to +5V to avoid the overvoltage condition.