level switch high high

Dear all,
i have LS1200 murphy used for suction scrubber (connected NC contacts to 1756-IQ16 card) and the question is:
when i triggered switch manual by hand using AVO meter the contact open and when i remove my hand returned NC. but the strange thing if i swap the probes of AVO, the contacts still NC if switch is triggered or not.
and then when i disconnect its wiring from PLC panel all thing becomes normal.
what is the illustration of this issue...

If the PLC input has a surge suppressing diode across it your meter current may be going through the diode when the leads are reversed.

With the switch wired, if you set the meter to measure voltage and measure the voltage across the switch then when the switch contacts are closed you should read near zero voltage drop across the switch, and with the switch contacts open you should read the supply voltage drop across the switch.