If you use an external monitor, Big Sur has a tendency to hide windows off the edge of the screen when it wakes. macOS will do its thing when you click the doc

If you use an external monitor, Big Sur has a tendency to hide windows off the edge of the screen when it wakes. macOS will do its thing when you click the dock icon and bring the window to the front, but it can be hard to notice because only a sliver of the window will actually be visible.