TeacherMade uses the industry standard known as "Learning Tools Interoperability" (LTI) to send grades back to Canvas. Unfortunately, the data we're allowed to send is very limited, and we can't tell Canvas whether we're updating a score because a student turned in their assignment late or because you were editing it. So, Canvas assumes it's the student and marks them late. We're investigating to see if there are ways to avoid the issue, and are sorry for the inconvenience.