Baked a cake and it had a perfect 2 inch border all the way around. The center was done, but was a lighter color and much smoother. Baked in a 9x13 metal pan. Ive never seen this before. Taste was unaltered
When I bake sheet cakes, I ALWAYS use an insulated baking pan. Mine are called Air Bake. The insulated pans keep the edges and bottom of cake soft, more evenly baked, without those hard corners, you could also place a couple flower nails spaced out to help the center cook more evenly.