by Ailsa Enting-Hawke.
Hi Alicia,
I was having the same issue and I haven't found a way to fix it directly but I found a workaround that may help you.
However, this will only work if you don't mind losing the student submissions and getting them to upload them again. (You can download all submissions first)
If you backup the course then restore it with no user data for that assignment then it will revert the submissions back to zero.
Luckily for me these were only all tests so I was able to do this.
Good luck!
Ailsa