Hello, Uwe.
Totally understand your concern; nothing can be worth ruining your work in the end with such blemishes. I did the preliminary test on the timber I use. I this case it is Cherry. The results come OK. At least for a given (immediate) time. I don't know how it will react over years, but the past few weeks, I have not seen any signs of a reaction. Below are my tests with few coats of Linseed oil.
I am suspecting those pins are stainless steel (they don't stick to magnets). Also, I do not blacken them with chemical solutions only anil the head for 'blue steel' effects, and I will not use stain for this build, only Linseed oil. Also, I don't use glue for pins, I push them like nails. Maybe the glue is the one causing trouble? Technically, stainless steel shouldn't be oxidized and react to any chemicals. Time will tell...I guess.