In article , "Michael Koblic" wrote:

There is something about doing the final wash in tap water: It seems to
promote oxydation. I managed to avoid it to some extent by doing the final
washes in distilled water. At least that is my personal impression. I have
not found any literature reference to this phenomenon.

Possibly due to substantial aeration of the water as it comes out of the tap,
vs. minimal aeration of distilled water being poured from a jug.