I have been making lemon bars for a while and have never had any problems. My Last 2 batches in the last week have both come out with the filling on the bottom of the shortbread base leaving me with upside down lemon bars. I was wondering why has this happened and 2x back to back. I have not changed recipes and I am not doing anything differently. My shortbread base is cooled, overly cool maybe as both times I let them sit for over the normal 15 minutes, could there be a magic window for when the filling needs to be added?