There is a galvanic reaction that can make the brass white on the surface. Too much citric acid or pins that were not cleaned of contaminants after the previous tumble. Someone else reported it here once, so you could try searching for it. I can't recall what it came down to. May have just been metal dust on pins that hadn't been tumbled and dulled a little previously. May have been a steel or a nickel-plated case that got mixed in with the brass. I just don't remember.
Anyway, an easy way to tell if that happened is to put the pins in your old vibratory tumbler with some polish and see if they turn brass-colored. Whatever is plated on them will be thin and should come off that way. Polish one by hand with some tumbler polish or some toothpaste as an alternative. See what you get. Try using a magnet to check for steel cases, too.