This issue seems to occur only on 30. The weird thing is that master and 29 seem to be working okay.

Steps to reproduce:

1. Create an assignment (no need to set submission types).
2. Under Feedback types, mark Feedback comments and Offline grading worksheet as checked.
3. Click Save and display.
4. Click View/grade all submissions.
5. Grade the student. Enter a grade and a feedback comment then click Save changes.
6. Click Continue to go back to the grading table.
7. Select Download grading worksheet in Grading action.
8. Open the downloaded offline grading worksheet CSV file (using Excel or any text editor) and modify the grade and/or the feedback for the previously graded student then save the file.
9. Back to the grading table, select Upload grading worksheet.
10. Upload the previously modified CSV file.
11. Instead of a confirmation message similar to Set field to ..., the page displays No modified grades found in uploaded worksheet. This makes the teacher unable to modify the students' grades using the offline grading worksheet.
12. If the teacher adds a grade/feedback comment to a student that has not been previously graded, the confirmation shows up but only indicates the setting of the grade/feedback comment for the newly graded student. Clicking confirm only sets the grade/feedback comment for the newly graded student, but not the modified grade/feedback for the other previously graded students.