Other switches on game wouldnt cause an issue as matrixed switch columns and rows are not part of the trigger switches for special solenoids. Sounds like you may have some debris on the trigger switch for that bumper. It's the one that the round wafer closes when a ball hits it. That's why disconnecting orange wires causes the issue to go away.
It could be the little blue cap on the switch stack is bad or a solder splash on the switch stack.

When rebuilding upper flipper you may have had a solder splash fall on that bumper switch.

I always replace those caps on the old Williams games now. Makes the game more snappy and fast as the bumpers and slings will be more responsive.