Is that even a thing? Drying too long? That's a possibility because I did have them in the dehydrator longer than what I usually do. I think my dad might still have a cookie so I'm going to go over there and take a picture.
Yes, definitely. I've had the entire layer of icing come off cookies in big "sheets" (which is why I call it "sheeting") when I've stored cookies very long under dry conditions. Same thing can happen with over-dehydrating.