What I did:
Tried to move a collapsed category in between or after other collapsed categories.
What happened:
It would only try to make the category a subcategory instead of keeping it as a top-level category.
What I expected:
I wanted to simply change the order of the categories, by drag-and-drop.
Things that might be helpful to know (Agenda version, OS and model, etc):
When I exanded the other categories, then it would let me rearrange them as categories without attempting to make them subcategories.