Mysterious Problem?

Yes, these other inputs were needed in the past.... and worked fine. The machine doesn't use ANY 2-wire sensors And YES! I hate them too.
The machine underwent a mechanical modification to make room for another Production line to lead to it.
Inputs 14, and 15 were no longer needed. 12 was removed also, but later added back. These are 4 wire Keyence emitter/receiver sensors....paired into a common M12 connector.
I:1/12 is electrically and 'addressably' the same as before. It even worked for a week after it was added back.
BUT......... NOW it needs that 2nd common on the lowest terminal.

I have had in the past prox sensors that the dummy light turns on and off and everything looks like it is working correctly. But when I monitor the input in the PLC to see the actual input coming on and off it was not.
Sometimes the switch will get damaged and stay in an on or off state, even disconnecting the cable and reattaching, it is still stuck in the same state. If it were me, I would try to replace the prox switch first before replacing wiring, unless for certain a damaged cable.

As long as it works with that second common... which from what I can tell is what AB intended, I don't see a problem... though I understand your frustration and confusion.

I just had to replace 2 wire sensor in a transport cart. Wanted so much to put a normal, 3 wire sensor in instead.

But the cart didn't have a 0V lead going to it at all. No spare leads in the cable through the flexible duct either. The cart was wired to sinking inputs, but the valves in it were from sinking outputs - so the only 0V was the signal to each valve that I couldn't use. The ground was from the servo controller output for the motor & the encoder cable run direct without any terminal connections to try and find 0V.

BTW - I also hate 2 wire sensors, except maybe cylinder position switches.