Hi @John_Gaspar,
Since nobody else has answered this definitively, I can offer a possible explanation of what's going on...
We've been told in the past that quiz submissions are actually a function handles by the user side in the browser and/or a Canvas mobile app. What this effectively means is that if there is an open Canvas quiz attempt, it will sit somewhat "in limbo" as an open attempt after the accept until date (or after the timer runs out) until the student next logs in to Canvas. When the student accesses Canvas at that point, the system should realize the quiz needs to be submitted and will do so at that point. As a Canvas admin, I wish this worked differently, as it causes a lot of confusion around the quiz submission date (it will appear students got more time/submitted after the accept until date/time, but they really didn't.
So for your situation, you'd need to ask the student to log in again and the quiz should get submitted. I'm fairly confident this is the solution, but not 100%, so please do post back and let us know how things went!
-Chris