How to reproduce:

1. Create a small course using development tools
2. Edit one of the assignments and enable submission (text for example)
3. View the gradebook (ex: /mod/assign/view.php?id=XXX&action=grading)
4. Order by 'Last modified (submission)' descending (triangle pointing down), it is ok as we have no submissions yet.
5. Login as a user and send a submission.
6. Go back to view gradebook, last page, it should be there the submission as expected.
7. Login as yet another user, go to the submission page BUT DO NOT SUBMIT.
8. Go back to view gradebook, last page, the user will be shown there as if it has submitted something. It happens because when he/she visits the submit page it creates a record in the {assign_submission}

   table with status = 'new' and a timemodified.

I would expect it to only consider if status='submitted' as implied by the column name "last modified (submission)".