The issue is with Step 2 (highlighted below), i'd ensured the close date had expired but the time limit wasn't reached, but HADN'T logged in as the student yet, however the status here https://qa.moodledemo.net/mod/quiz/report.php?id=30&mode=overview changed from 'In-progress to a completed timestamp regardless.

Test Steps:
1. As student, start a quiz attempt and answer some questions but not all, then log out of Moodle.
2. Wait until the close date has expired but the time limit has not, which must be BEFORE the quiz cron runs (hourly), then log back in as a teacher and make sure that the attempt is still shown as In-progress.
3. Then log in as a student, and go to the quiz info page. This should cause the quiz attempt to be automatically submitted, so you should see that your attempt is finished, with those questions you had answered graded.
4. Log in as a teacher and check that the attempt is no longer "In-progress".