We are running Moodle 3.4.2+ (Build: 20180515)
We have had several students where all (or almost all of their grades) in the Grader report disappear, in all their classes, not just one class.. Their grades are also not visible in the other views, user report, single view. On some of the grades, if you go to the individual assignment, you can still see the grade there, but with other assignments no. Maybe that is related to how the instructor graded.
In one incident, the Participants list said the student had never been in class (not true) so I thought maybe the student is getting unenrolled from our Moodle system and then put back in again. (We have a new software bridge that brings the enrollment info in from our Registrar's Jenzabar system to Moodle.) But the setting is to recover grades if student removed is on, and I tested that and it works
Or I see many bug fixes out for 3.4, but none that seem to talk about this issue.