Assignment - sorting by " Last modified (submission) "

When an instructor is checking assignments and wants to sort by "Last modified (submission)" the sorting does not always work properly. This tends to be an issue if not all students have submitted or in some cases, they did not attach a file which is technically "no submission" yet Moodle seems to count clicking the "submit" button as a submission.

Description from instructor: "So, there might be five students listed first, who didn't submit anything yet, then the first student to actually submit, then five more students who didn't submit anything, then students who did submit, sorted in proper order, then students who didn't submit anything, then more students who did submit."

Screenshot: https://drive.google.com/file/d/0B4-Kvr0R1fMUVF9HZjItaWg3RjQ/view?usp=sharing

Version: 2.8.7