Discovered while looking at MDL-50612, I am creating a separate issue because it affects 2.8 as well:

During editing of the module we remove the 'Grade category' form field if outcomes are used and the category of outcome conflicts with the category of the grade:
https://github.com/peikk0/moodle/blob/MOODLE_28_STABLE/course/moodleform_mod.php#L136..L182

But the default value for the $gradecat is false (line 137), which causes the field to be deleted completely (see line 174) for the activities such as database or glossary when rating has not yet been set and therefore the grade item was not created.