if that's a correct date for when you opened it, it would be four months old so that could be the culprit. I know with Autumn I never completely finish a pen before her numbers go wonky. I usually get a solid 3 months out of one pen. And they've never looked bad, in fact the one I just found under the couch that I lost 9 months ago looks perfect but I'm pretty sure it's toast.
Mel and The Fur Gang