It's not just the upper edge but all long the right side as well. Because it's so regular and sharp, I bet something involving time in tray is responsible.
Try NOT stacking and see what happens. It could be a light leak in your darkroom and shadow from edge is causing it. Do you expose another print while previously exposed prints are in tray before they are well fixed? That could be an issue too, along with not-so-safe safe light.