I've seen that type of staining/ corrosion before. I had some 270 win brass that did the same thing. I use Brasso polish in my shell cleaner. So I always thought it was caused by the ammonia in Brasso. But have since wiped my cleaned shells lightly with CLP on a flannel rag after there cleaned and before their storage. Unloaded empty's and loaded ammo. Both. Haven't seen that problem occurring anymore. I'm starting to think that staining is caused by what small amounts of powder /sulfuric acids remain on the brass even after there polishing and humidity while in storage may be the cause. Since brass is a non-conductant. The staining couldn't be caused be electrolysis. So it has to be some sort of chemical reaction causing the problem. I've never heard of that staining happing to those having pin tumblers? I suspect its their soapy cleaning solution that removes the acids well enough in that scenario.