When you say 14 and 13.7 volts, then there pops up a color in my head, Gray-Yellow.
That is 12V unregulated from J116/117/118-2
Possible contact between Gray-Yellow (12Vunr.) and White-Gray (sw.row 8 ) wires !?

You could disconnect the 12Vunr. playfield supply, located on J116,117,J118.
Leave the one with 3 thick wires in place and remove the other 2.
Do you still have 13.8 volt on White-Gray ?

This way you may have a direction to search for.
-If it stays 13.8 Volt then have to search for an other cause.
-If its gone, have to follow all Gray/Yellow wires.

Interesting situation, the way you describe it sounds like the problem is indeed a connection between 12Vunr and switch-column8.
But if the voltage comes back to normal on C8 when the one with the thick wires is disconnected (display goes OFF?), the short may already be found in the backbox.

If visual inspection doesn't give any results, then taking measurements is next. Maybe this will give some findings:
-Machine 'OFF"
-Multimeter in 'BEEP' function. The DMM has to beep instantly if a short occurs. (some multimeters need a few tenths of a second to react)
-Clip one pen to a Gry-Yel 12Vunreg wire . ( this was likely the source of the problem ?!)
-With other pen, measure all white and green wires of switch matrix, but also all metal parts (not the grounded ones). wiggle with the wires.

Can you take a close-up picture of the 10-opto-PCB + wires? Also in surrounded area

nice pictures

No, that is not what I meant.
I think there is sometimes a connection with 12Vunregulated and the switchmatrix because you did find 13.9 Volts on the white wires. The white wires (rows) can handle some overvoltage, but once a switch is activated, the column drives are under heavy load. That also explains why it is not always the same column.
So, I meant that you want to measure if there is a direct contact between the 12Vunreg and white wires from the switch-matrix.
Done by turning machine off, one pen of DMM on gry-yel and with the other pen all the white wires.
(btw. not the gry-yel from the flipper coil visible in some of the pictures.)

I took one of your pictures and marked 3 areas, 1 with a damaged lamp matrix wire (red) and the other 2 areas with harness bundled together where due to stress and possible sharp edges of the stands, the insulation of the wire can be cut and connecting each other in a wrong way.
This is the kind of short that do happen sometimes and are difficult to find.

picture (cropped):
tz-short.jpg

Not exactly, but close.
Yes, 12Vreg is used on CPUboard by the 339 comparators and the uln2803.
And the 10-opto-board under playfield is powered by the 12Vunreg.

.......ah, you know what, a picture will explain.
abc.jpg