I have noted that the Arduino analog inputs can sometimes pick up signals from adjacent channels.
You can try pulling the analog signals to ground with a 10K - 100K resistor to see if it corrects the unwanted readings.
I just wanted to confirm that reducing the "Max. requests" number to 1 allows multiple Modbus reads of the Click PLC.
I agree that reading registers in groups of 4 or 8 is more elegant and will try that next.