Moodle version: 4.5.1 (Build: 20241209)

In our Moodle installation, the question bank contains approximately 45 categories directly under the system-level category. These categories are widely used across multiple courses, allowing for more efficient management and updates. The questions are linked to quizzes that display them randomly.

We have been using this setup since Moodle 4.1.3 without issues. However, after upgrading, when creating a course backup, all questions from all system-root subcategories are being copied into each course. Additionally, the copied questions are duplicated excessively, resulting in questions.xml files several GB in size, containing records of up to 10 million questions.

This issue makes the system inoperable, as the restore process becomes unmanageable due to excessive data, leading to system crashes.

When testing on a fresh Moodle installation with minimal courses, the backup process works. However, in a real scenario where multiple system-level subcategories exist (in our case, around 45 subcategories with 50 questions each), the excessive duplication prevents both the backup process and normal question bank operations.

Steps to reproduce:

1. Create multiple question subcategories directly under the system-level category.
2. Create a course with a quiz that randomly selects questions from one of the system-level subcategories.
3. Perform a course backup.
4. Download the backup file and inspect the questions.xml file.

Possible related issues:

https://tracker.moodle.org/browse/MDL-75854