1. Create an empty project and save it as a single .capx file.
2. Add a sprite to the layout, and draw something in the editor for the Default animation
3. Close the animation editor, and save.
4. Open the animation editor for the sprite and rename 'Default' to something else.
5. Close the animation editor, and save.
6. Open the animation editor and notice that the sprite editor now only shows a small dot in place of the previous drawing for that frame.
EDIT: Found a workaround. Save the project after renaming an animation, and then close and re-open the project. The frames should be restored.