I just experienced this issue using 2.5.
I used the import function to move a single book resource from course A to course B. When the book resource arrived in course B all the questions and categories from course A had been added.
This is a disaster as both courses have thousands of questions in hundreds of categories stored at the course level. Many have similar names which will make it almost impossible to manually delete the accidentally imported questions/categories. I'm now looking into restoring the course from a backup; but since I imported the book resource weeks ago without noticing the error all recent work will be lost.
I really think this is something developers need to look into. The import function seems like a very simple and intuitive tool. But wow - so easy to totally mess things up.
If it is not viewed as a bug then there should be something to warn you that you are about to import all course-level categories and questions even if you are only importing a single resource that is in no way associated with the question bank.
Or, at the very least, the Moodle Docs page should clearly explain the situation. I carefully reviewed the Moodle Docs before I did the import. It's amazing that nothing is clearly mentioned about this issue. It does say that "Importing a quiz from course A to course B, will also add the questions to the question bank so you can use them in new quizzes." But it seems reasonable to ignore that if you are not importing a quiz.
https://docs.moodle.org/25/en/backup/import
Thanks,
Lael