I would love to hear what the manufacturer has to say on this. High purity copper, even when exposed in the average climate controlled home, should not suffer the fate you are describing. Either the quality of the copper is in question or there is an abnormal reaction occurring with the insulation. In either case, not normal and not acceptable.
Corrosion
On a Shunyata Diamondback power cable I use, which has clear insulation, there is green oxidation visible on parts of the shield. Can that have an audible effect, is it a concern?