Maybe that bit of tarnish/rust has a coarse texture? That would powder the strop a bit. If you were very careful and removed the tarnish the problem might go away. Perhaps stropping on a balsa paddle with some metal polish rubbed into it and left to dry might smooth it over a bit.

We used to get specks of rust like that all the time - the old water main was cast iron, and each time a new service was taken off it, they drilled into the cast iron main and the tiny filings got into the water supply, dried on metal, and left little spots of rust - even on stainless. Doesn't happen now the main has been replaced with plastic.

Regards,
Neil