Well, that would be a bug, but to be able to do anything about it, I would need more details about the exact set-up that leads to this.
The automated test for this plugin tests that you can grade when logged in as a teacher: https://github.com/moodleou/moodle-quiz_gradingstudents/blob/master/tests/behat/gradingstudents.feature#L73 - if you can decode that. That test passes. So the problem situation must involve some additional settings somehow.
The automated test for this plugin tests that you can grade when logged in as a teacher: https://github.com/moodleou/moodle-quiz_gradingstudents/blob/master/tests/behat/gradingstudents.feature#L73 - if you can decode that. That test passes. So the problem situation must involve some additional settings somehow.