The problem with chasing electrons are they are so tiny their hard to see.
They are also sneaky and can all join hands and look like they are happy but as soon as you turn your back they let go and start taking naps.
Then there are cases like yours where a perfectly good looking wire can decide it doesn't like the surface its supposed to be contacting.
These will often collaborate with their old buddy, corrosion, and build a barrier between themselves and some ground location or another wires connector.
Glad to hear you found their latest attempt to quit working.