Hi Daniel,
I think that I understand this better now...
I looked at two assignments on our system. The first has "File submissions" set to "Yes" under Submission settings. The other (the assignment for which the problem was raised, which was created only to allow feedback on exam performance) has both "Online text" and "File submissions" set to "No" - i.e., it doesn't allow any submissions.
In the former case, there is a "Filter" drop-down in the options section below the "View/grade all submissions" grid. This does not appear when viewing the assignment that doesn't allow submissions. However, it appears the filter is still in effect, and is filtering out all students (as no-one has submitted anything). This means that users could fix it themselves, if they go to another assignment that shows the filter and reset it, but only if they can work out what's happened.
Now I've managed to work out what's going on, I've actually found a Tracker bug report - looks like it's fixed in 2.3.5 and 2.4.2: