Platinum solder joins turning black

I made a platinum diamond solitaire ring a year ago with a rub over setting. Now the customer has come back unhappy as the solder joins are turning black.

I have made other rings with this medium platinum solder from Cooksons with no problems of discolouration. I also tested the ring with acid to check I had not used white gold by mistake but the acid stayed clear showing it was platinum solder.

Does anyone have any advice on this. If I rhodium plate it I presume this will wear off and the black will come back.

Thank you Richard

I don't use platinum that often, I have never had this problem Platinum is known for it's ability to resist tarnish, are you 100% sure you did not contaminate it accidentally, maybe the customer has exposed it to a chemical of some description, and yes Rhodium plating will continually need re-plating