After some rearrangement of windows on different workspaces, the Activities overview will not show any of windows on a workspace.

Here are the steps to reproduce:

1. Open 3 windows and put them on 3 different workspaces (one each), so you should now have 3 workspaces in total, with one window on each one
2. Go to the first workspace
3. Exit the Activities overview
4. Now enter the Activities overview and do not exit it until the steps say so.
5. In the Activities overview, switch to the second worspace
6. At the little boxes on the right side, drag the window from the second workspace (that you're on) to the fourth, empty workspace. This will cause the second worspace to disappear and a new one will be created at the bottom. So the previously third workspace will become the second now.
7. Repeat step 6
8. Repeat step 6 again
9. Remain still in the Activities overview

Now you will see that when you switch to the second or third workspace in the Activities overview, no windows will be shown at the center of the screen. The windows will still be shown on the right-side boxes and if you exit the Activities overview and enter it again, everything will be back to normal.