A quick way to test wiring interference without an scope etc… disconnect the probe wires and use a jumper to trigger the Rambo z probe pin directly. For stock mpcnc, pin to 5v is not triggered, pin to ground is triggered. Be ready so it doesn’t crash. If it works like that, you are likely dealing with bad endstop wiring. If not, it’s likely firmware.
If it’s firmware, I’d guess soft limits may be at play.