I've gotten a couple of reports to where students claimed they passed an exam, but it never ends up saving to their account. After digging around the code I've found that there's is zero logic for expired sessions.
What this means is, if a user is taking a quiz or exam, and their session expires, aka they leave the page open for too long, or swap wifi's (very often on mobile), then when they submit and finish the quiz it looks like they did complete it. But under the hood that grade will get stored as if a "guest" submitted it (because their session expired, so they were logged out).
Funny enough, quizzes shouldn't be able to be taken by guests, but there's no logic stopping a guest from submitting one. In one of my exams the guest user had over 400 quiz attempts, from all the random times different users sessions expired before hitting submit, (which is VERY common on any and every type of site).
With the extreme lack of support and updates with Guru I'd recommend people start looking for alternatives to Guru because I doubt a fix for this will be shipped anytime soon considering they don't even support Joomla 5.4