Hi Naomi, @Naomi p
I have a recipe for lemon stars. In this recipe, the applied RI is also baked. In fact only baked for a long time and lukewarm so that it doesn't burn. So the cookies are not baked properly. They taste great, but these cookies actually tend to lose the RI. In this case it looks like the cookie, which is still "oily", does not connect properly to the RI. Maybe that will help? Maybe the side of your cookie was very fatty? I'm looking for a picture once because I had baked these cookies for Christmas too. Maybe you can see it ...?
Or it could be your icing (above) was over-dried (due to the baking of it), which caused it to separate from your cookie. This would be more consistent with my experience that: as cookies sit and the cookie/icing dry out, it's more likely that sheeting will occur.