When using the 'Write category to file' option in Question bank -> Export, sometimes the full set of parent categories is not exported.

Steps to reproduce:

1) In a course, create a Level1 category below top, a Level2 category below that and a Level3 category below that.

2) Add a question in the Level3 category.

3) Export Level3 (as Moodle XML with 'Write category to file' checked) - as expected, the file contains 3 category questions: Level1, Level2 & Level3.

4) Add a question in the Level2 category.

5) Export Level3 again - the file only contains 2 category questions: Level1 & Level3.