I dont think you changed the default. You just set a custom one.
This is just my observation, because i had also wished for this feature in Feedback center.
If you are using a Tiled Layout and saved that as default, AB will open with all the charts in that tile fashion. ( by using Tile vertical or Tile horizontal or dragged charts yourself)
If you close AB in this way, the layout gets saved as it is except the Analysis windows.
If you maximize the windows within the frame, then you don't have any tiles and the layout will be like the default shipped. here, all are full-sized frame windows that you see one after the other from the MDI tabs.
Maybe you can check this.