Wow! Now do I book in and take all 27 devices for checking, or… 
It must be carbon build up because I scratched the black junk out carefully with a sharp pin. This did the job, but I imagine that doing so wore away the gold plating, so I will probably regularly clean the pins every other week or so. I’d take pictures of my success, but my iPhone is currently under a backup restore.
I had a feeling that pin #4 on the lightning connector was power, because I couldn’t imagine why every pin was intact but just one. After all, I’ve had to clean light switches and other electronic switches and connections because of electrical carbon build up.
Weird too that two of my battery cases went dead on me and my mother, but I also noticed that, sure enough, pin #4 was black on each one. 
It’s also weird that pin #4 burnt out since I never took the phone out of the case, but I’ll be sure to clean them now with my sharp needle pin…those cases were expensive.
I’m relieved to know I’m not alone wondering why one specific pin is burning out. 
My son was complaining his 6S was no longer charging. So I thought I would have a look inside the port with a torch. The amount of fluff in there was incredible. I was able to clean it out with a paperclip. Boys are filthy creatures.
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I’ve had to do that on a few phones now. Built up pocket lint compacted by the lightning cable over and over again. Wireless charging minimises the compacting part now 
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