Issues with assignment submission

I’ve started using github classroom this semester as I introduce students to git in an undergraduate R-based stats course. Its been going well but I have a question about submissions that I hope someone can help me with.

All my assignments have due dates, but only some show as “submitted” (as opposed to “not submitted”) after the due date. I have a test student account and haven’t been able to replicate the issue. I’m assuming its an assignment issue, as all students who accept the assignment end us “submitting” or not. My only thought has been it might occur when I edit the assignment to add the due date instead of setting it when I first create the assignment.

Any ideas about why this is happening? It doesn’t impact my grading but I’m curious.