Comment by ErroneousBosh

In the olden days, when things like TVs and VCRs had cabinets made from polystyrene and PVC-jacketed power cables, you'd often find that if something had been put away with the mains lead coiled up on top, it then left a "scar" on the case. This is because the plasticiser in the PVC jacket attacked the polystyrene, leaving the mains lead fragile and brittle and a nasty gooey mess on the case that you couldn't fix.

That's why the leads are wrapped in a polythene bag.

When people started using polystyrene sheet insulation in houses (thankfully they no longer do this!) the cables running inside the walls were affected in the same way, with the PVC insulation rotting off as the plasticiser leached out and attacked the polystyrene. Of course there you had the added joy of having a potential electrical fire with a source of just-about-inextinguishable fuel, the polystyrene foam made of fuel and air.

I wonder if something similar has happened here, something's gotten onto the fibre jackets and pulled the plasticiser out?