@probinson1
Enhanced rubrics might be the cause. I have it turned off. You could turn it off for the course, test the rubric, and then turn it back on. That may not fix the issue as I suspect it's something messed up with the database itself (especially since it works with other discussions).
There has been a lot of headache recently about discussions and how messed up they are. If your institution had checkpoints enabled briefly around January 20 and then they got turned off, some submissions made during that time are missing. There's a whole thread on how bad it was. It was more than just what I wrote with so many oddities happening that your case could easily be another abnormality without realizing it.
Then another situation happened to us that I found out about this weekend. Canvas had a known issue that if you imported course content without dates or shifted dates that it wasn't creating the submission records. Their known issue was with classic quizzes, but we're seeing it much more widespread than that. Without submission records, you cannot see the student's name in SpeedGrader and some students may not be able to submit. Canvas fixed the issue of the import not creating the submission record on January 15, but they didn't go through and create the missing ones. We had 92 courses, 462 assignments, and 6919 (those are all approximate) submissions with missing records.
One way to fix that is to go through and unpublish/republish the assignment. You cannot do that for an assignment with submissions, so another way to create the records is to edit the due date for the assignment. A third is to go through and run this script that I cannot find documented anywhere, so it may be something that Canvas support has to do.
None of that is conclusive. Here's where I'm at now.
- There might be something messed up on the backend with that particular assignment. You can try editing the due date and see if that fixes stuff.
- It may be related to the discussion checkpoints and I don't know if there was a fix given in that discussion but there were a whole lot of things not working right.
- I don't know if enhanced rubrics have anything to do with this. I've disabled them as they break my entire workflow.
It may be that for now, you just have to manually transfer the grades for this assignment and then it will go back to working in the future.