Discovered by andelacruz while testing MDLQA-19799

When a quiz is restored from one site to another, questions originally stored in the quiz's private question bank (e.g., "Default for Quiz 1") are unexpectedly moved to a different question bank in the restored site.Steps to Reproduce:

Steps to reproduce

1. Create a new quiz in Site A.
2. Add new questions directly to the quiz, ensuring they are stored in the quiz's private question store (e.g., "Default for Quiz 1").
3. Backup the course containing the quiz.
4. Restore the course to a different Moodle site (Site B).
5. Check the question bank associated with the restored quiz.

Expected Behavior

The restored quiz should use copies of the original questions that are stored in the new quiz's private question store (e.g., "Default for Quiz 1" on Site B).

Actual Behavior

After restoring the course to a different Moodle site, the questions are no longer stored in the quiz's private question store but appear in a different question bank instead.