I think I figured out what #16AF was taking about.
For reference: if a featured image is hidden by filters, the Featured Image box is absent from the front page. If a featured image is hidden by the eye icon, the next most recent Featured Image is displayed on the front page.
If, for some reason, you’ve hidden (via the eye icon, not via filters) every featured image back to the point where one was deleted, you’ll see this:
…and, unlike for non-deleted images,
(I can only guess #16AF really likes having some old featured image present on the front page…?)