I noticed that my Santini Italia Calypso (piston-filled) fountain pen, which is fitted with an 18K gold nib supported by an ebonite feed, somehow wrote in a very different and, counter-intuitively, much lighter instead of darker colour, after then pen was capped and unused for about a day and a half. The nib unit has just come back from Italy for repairs, and the pen was freshly filled with Waterman Mysterious Blue ink, which I only bought a couple of weeks ago from a local department store. The pen does not have a history of allowing ink to dry out quickly while capped.
I decided to empty the contents of the reservoir (into a sample vial), flush and dry the nib unit, and fill the pen anew from the same bottle, to see if the issue persists. (Spoiler: It does.)
Chromatographic comparison between the ink in the pen's reservoir after the weird lightening has been seen, and the ink still in the bottle from which the pen was filled in the first place, can be found here:
The retest results can be found here: