If logged in as non-editing teacher, go into assign grading page (ensure that submission comments are on, e.g. no team submission, etc), and try to expand submission comments. Nothing will happen, while you will get the comments expanded being logged in as a teacher. It works on other view (grading particular student, etc).

If you take a look on the browser's console, you will notice this:
Uncaught TypeError: Cannot call method 'on' of null at module.js:153