I've been downloading the CSV file to do my grading. Recently The "Student Name" field changed from the "display name" I've assigned to students to "username" or something else (I'm not exactly sure).

Why did you change this? Can you please change it back?  I don't understand why you would be changing that field? Why not add a column instead of removing the key identifier of the CSV data....

That csv file is useless if you do not keep it consistent.