I can confirm that this also happens in 4.2.15 Also see the test I attached which basically asserts what was written in prosa before. Here is a little workaround. You could instead of using distinct also use a group by, but you probably already know all this.
SELECT DISTINCT s.id, q.id, sa.answer
FROM Student s, Question q
LEFT JOIN s.studentAnswers sa
WHERE sa.question = q