Quiz statistics report ignores tags when analysing random questions.

Steps to reproduce:

1. As teacher, in a suitable test course.
2. Add a new quiz activity called 'Test quiz'.
3. Navigate to the question bank for that quiz.
4. Create a new Question category called 'Random question pool' inside the category 'Default for Quiz: Test quiz'.
5. In that question category, create two TF questions, and give them both tag Q1.
6. In the same question category, create two Multi-choice questions, and give them both tag Q2.
7. Go to the Quiz 'Questions' page to build the quiz.
8. Choose Add -> Random question.
9. Select to add a Random question from existing category 'Random question pool', with tag Q1.
10. Choose Add -> Random question.
11. Select to add a Random question from existing category 'Random question pool', with tag Q2.
12. Now, as four of five different test student accounts, log in and attempt the quiz.
13. As teacher, log in, and go to Test quiz -> Results ->Statistics.

Expected result: Since the random selection of actual questions for Slot 1 and Slot 2 of the quiz are different, then they should be analysed separately, with a separate summary row.

Acutal result: the actual questions that appeared on both places are lumped together in the analysis, rather invalidating it. You can tell this becuase there is only a single "Range of statistics for these questions View details" row.

Here is a screen-grab, which does not exatly match the steps above, but is similar (this quiz has 10 questions.)