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Cobalt magnets tarnishing

I’m using some Samarium Cobalt (SmCo) rare earth magnets insome of my
jewelry designs. I have them triple plated for stability in nickel,
then copper, and finally a third plating of gold.

After a year or so, the gold is tarnishing. I’ve used Neodymium
(NdFeB) rare earth magnets for some time, with the same triple
plating, and the final gold plating stays beautiful.

My supplier says it must be a reaction with the cobalt. I’m skeptical
about that answer, and wondering if the plater put such a miniscule
layer of gold on that the underlying copper is oxidizing.

Any thoughts on this? Do you suppose it could be a reaction with the
cobalt like the supplier says? Or something else causing it?

I appreciate any possible theories, explanations - and especially
solutions - and thank you in advance.