I have been trying to manually enter grades into a course in Moodle (3.0.1), using both CSV and XML export/import. (As the documentation describes, I start with a file that is exported from Moodle.)
I do the following:
--- Export a courses grades to Excel. (I have attached the resulting XSLX file.)
--- I modify the Excel, changing a couple of grades. (See the resulting CSV, attached.)
--- I import back into Moodle. Moodle states that the import is successful.
--- No grades are modified.
I also went thru this same exercise with XML, to no avail. (See attached XML file, that I imported.)