I use the "Has ungraded submissions" filter, so that my Gradebook is not cluttered with hundreds of quizzes that have already been graded or students haven't encountered yet.
The "Has ungraded submissions" filter works great, until I refresh the page or come back to it later, in which case the entire Gradebook is blank... but the filter is still on (see attachment).
I'm sure I must be doing something wrong, because that doesn't sound like a feature!