Our tutors have occasionally observed cases where they graded a group submission and provided feedback (in the form of PDF annotations or a feedback file), but neither grade nor feedback was saved for some users in the submission group.
This happens very rarely and we haven't been able to replicate it on a dev machine. So we couldn't debug to find out what's going wrong.
E.g.: In one case we had a submission group of size 3 and the grade/feedback only got saved for two of them.