Same problem here, if a student can submit non behalf of the group then the completion status should behave as the submission status. The fact that I can grade the submission but not being detected as submitted doesn't make any sense and I had to mark completion manually.
Any clue for a workaround or solution?
Thanks,
--sergio
Edited: there's an open issue on this, apparently the bug resurfaces every now an then because there are analogous closed issues for previous releases.