There’s an issue affecting many timed quizzes (where lowest time for a given score is the best score for that quiz for a user) where the fastest time isn’t always showing. Working now on fixing this, should be good soon. Also – all scores are being recorded so your faster scores will be displayed when this is fixed, they had been saved.
UPDATE – this hopefully is fixed now. Let me know if your fastest score isn’t displaying for a timed quiz (keep in mind the higher scores take precedent over faster times).