I've just encountered the same problem. I created a no submission assignment to hold a final grade for a test that is computed as a sum of multiple components. The column in the grade center is empty. I exported the grade center into a Libre Calc, computed the grades for that column, saved as .csv and imported into Canvas grade center. The response every time:
There were no changes detected in the gradebook you uploaded.
I tried erasing all the other columns, no difference. I double and triple-checked that I had actually saved the csv file. I even tried exporting again, and copying the column I had computed into this otherwise untouched, fresh sheet, and saved. Again the same result.
I've done this same thing many times in other courses, I've never had this problem before.
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