When I see weird things like this I start looking at pots and jacks. Have you swapped the input jack out with a new one? I've seen the inner sleeve of an input jack have lots of dirt and debris coated on the on metal. As part of my repair process, I use a Qtip with isopropyl alcohol and wipe the inside sleeve of the input and output jacks. I'll do the same for the tip of the jack.
Many guitar players grab the sleeve and tip of the guitar cord and transfer the dirt and oils from the fingers to the plug on guitar cord. Then this dirt and debris will stick to the inside of the guitar pedal jacks. I see this on old vintage units... not so much on newer pedals.