Reproduction steps:
1. Open attached project
2. Open scene "Test"
3. There is a Canvas with a Panel in the scene
4. Add a few Image prefabs as children of the Panel
5. Notice that Images stack on top of each other even though the Panel has a Grid Layout Group
6. Remove Scroll Rect from the Canvas.
7. Remove all of the children objects from the Panel
8. Add a few Image prefabs as children of the Panel
9. Notice that without the Scroll Rect Images are laid out in a grid