If authentication is expired when an assignment is submitted, upon successful exit from the re-authentication the assignment submission should complete. (And upon failure, the assignment should be preserved as a draft.)
If a lesson is left on display overnight, it appears that authentication expires in the background. However, the "Start Assignment" button succeeds (the form is generated locally, apparently). If the assignment is a text input, there appears to be no server-side interaction until the assignment is submitted, because the expiration is not detected.
If required, re-authentication will occur when the assignment is submitted, redirecting to prompt for credentials.
However... upon completing the re-authentication, the UI returns the student to the lesson and does not complete submission of the assignment. The assignment text is lost and is not preserved as saved content when the student re-starts the assignment. i.e., the assignment has to be re-done from scratch.