What happened here?

It looks like the copper is coming off the parent metal on the left and the rhodium clad is holding up on the SAME parent metal on the right. Brass, iron, nickel they electro plate it with what ever YOU’LL buy.. Just a cheap knock off, with a bad plating job.

NO they both didn’t start out rhodium plated, just the one on the right..

Regards

Sorry OP that's NOT what the picture shows. There is an alloy under the copper splattered on the clear insulation on the left, it sure doesn't look like copper. It looks like a knock off like I said. Rhodium will stick to plastic. Copper will stick but you have to prepare the surface.  Just because it what suppose to be made of copper doesn't mean it IS. I mean it LOOKS like iron or nickel or something else other than copper like on the clear insulation.. I see no rhodium,,

You ask I answered.. From what I can see. I have no idea WHY you would use rhodium. on a hot (at all) and surely not on the common or ground EVER. Its is for corrosion ONLY. Not a great conductor over copper. If it were submersed in sea water.. That would be a place to use rhodium.. WAY to hard.. It's used to help copper hold a spring BACH element too..