When a question category is moved from one context to another it is automatically renamed to "Default for <context name>" and creates a second top level category. This then breaks the category page stopping you from being able to move categories around.

Steps to reproduce:

1. Log in as admin
2. Create a course (c1)
3. Create a quiz activity (quiz1) in c1
4. View quiz1
5. Open the question bank from quiz1
   • edit cog > question bank
6. Click the "Categories" tab
7. Add a course category to c1
   • It's parent category should be set to "Top" underneath the c1 section
8. Use the up or down arrows next to the new category to move it either into the "Question categories for 'Quiz: Quiz 1'" or "Question categories for 'Category: Miscellaneous'" sections
9. Once the category moves into a new "Question categories for" (i.e. new context) it will be incorrectly renamed as "Default for <context name>" and the category page will no longer work (turning developer debugging on will show an error message)