These inks are incredible stable chemically. However, the pigment and carbon in the ink does settle more quickly in older inks that are near or past expiration and this could increase variability of the ink hue-wise. Not a particular shade over others to my knowledge but I will check our LOT #s when I'm back at work. I'm out sick at the moment.
That said, we use all of these inks every day in a humidity controlled environment at Cone Editions and do not have hue changes.
The world of inkjet printing is just as physical and variable as the world of darkroom printing. I like to remind people of that. Case in point is personal . . . long ago I started an editions printing company that sold itself as being able to create single-prints of an edition from start to finish over ten years. But paper companies came and went, printer versions upgraded, coatings changed, mills changed their water, ink manufacturing and carbon grading changed (often for the better but still), shipping changed, pollution changed, my lab changed locations a few times, I upgraded my lights! There are so many small factors that can contribute to a small shift in hue between one print and another that I frankly had no control over that and I finally had to re-educate my clients to either have them print an entire edition in one go or be ok with small natural changes from print to print. Often they would actually like these small changes. It's the old taoist idea of the "uncarved block" and is true of all art forms.
We test LaB values for every piezography ink before we manufacture and sell it. Our tolerances are way way tighter than even epson OEM because the nozzle frequencies needed to print Piezography are so much higher than Epson. A small shift in a light ink's hue will be immediately noticeable with Piezography because there is three times as many nozzles firing that light ink.
Long story short, inks are old. Possible printer is old. This can all add up to a change.
ps: Need to keep humidity between 35 and 50%.