A student S is explicitly assigned the global role 'student'. I have created a quiz which is all essay questions. S has taken it (this is a test site) without problem and his answers are shown alongside the right answer as expected.
When login is admin and go to the quiz with a view to manually marking the essay questions I find I cannot. The Quiz->Results page says that 1 student has made 1 attempt the test and gives the information about his score. When I click on the Manual Grading link I'm told:

The following questions need to be graded manually
No attempts have been made on this quiz

If the global role assignment is not made the issue does not arise