Here's what was reported to me and remains a real puzzle so far.
Under Moodle 2.8
A quiz is available between April 19th and April 26th. It lasts for 3 hours and is set so that attempts are sent automatically when time expires. It has 23 questions: 18 are manually graded and 5 are automatic (true/false). Nothing too exotic so far.
The teacher says that some of the students answers were lost. It really looks like it's true. It seems that the problem occured when he marked the manual questions this mornings.
Now, the only unusual thing I observed so far is that the "Attempt finished" time differs from one question to the other in the same attempt. As you can see in the capture below, 19/04/16 at 21:55 really makes sense. But 16/05/16 at 6:09 corresponds to the time the teacher was grading some of the question.
How in the world a same attempt (and there is only one allowed) could have two different finish dates?
Next step will be to restore a backup to confirm that all answers were answered. In the meantime, if anyone has a clue, please help me.