Interesting discussion. I too have had over the years, some wire where one or both conductors were patina coated, even a foot or more down from the end. I used to think it was the quality of the PVC jacket and that if it was poor quality ( as it was years ago with wire sold by the likes of Milspec Industries), but now I'll look at things differently.
An interesting point I will add is .... years ago, I had done a job at a condo development. The original builder ran 110 volt floods under every tree and used 1 piece of 12 / 2 romex for the whole run. NO conduit, no common sense.
Fast forward, we ran our low voltage lighting and got the system working. After it was operational, we had a couple of occasions where the lights stopped working in areas. Upon examination and troubleshooting, we found previously unknown line voltage splices which were in aluminum bell boxes and buried, which had failed.
For years, those splices sat underground unused. The connections corroded and degrade, though they still ohmed out on a meter, and carried power initially. Once the system was complete and running, the current draw through those compromised splices cause there to be arcing and subsequently, the splices failed.
Moral of the story- your system is only as good as the connections within.
And the second moral... don't trust underground wiring that you didn't put in or that is not in conduit so you could replace it. We never know what others do, how they wire, how they splice and where they 'hide' their splices. As much as you all can, on a job that has some existing wiring line or low, make it your point to sell the client on the importance of installing new wiring to insure the future integrity of the system.