Are customers skin reacting to platinum solder?

I’ve been using platinum solder for years for sizing and re-tipping.
I use extra easy or easy when re-tipping around diamonds and if I’m
sizing a ring that has diamond shoulders.

On a few occasions rings have been returned because the solder has
gone black.

It is quite a rare complaint but I’d like to know exactly how to
avoid this.

I know that easy plat solder has a silver content which is causing
the tarnishing but why is it so few and far between? Do I have a bad
batch of solder?

Are some customers skin reacting?