Thankfully, my BMS doesn't exhibit that problem. The three pictures below show the cell voltage readings, first with the sensor wires for cell #4 connected correctly, and then with each of the sense wires disconnected.
|All sensor wires connected, voltage read correctly
|Positive sense wire disconnected from cell #4. Cell alarms and shows "phantom voltage of 17mv.
|Negative Sense wire disconnected from cell #4. Cell alarms and shows phantom voltage of 17mv
I think this all comes down to the internal circuitry of the sensing device, and in particular how much impedance there is between channels.