Because of a glitch with a browser extension that I didn't catch until after posting this and another audio story chapter, the thumbnails were not appearing properly on either submission. I could have let this go, but it bugged me enough that I needed to fix it. I have since removed the browser extension that was causing the issue (which I was only using because it allowed me to see when someone posted a new submission on FA, even if I wasn't actually on that page), but, to fix the problem, I had to reupload the two submissions that I had put up since it was first installed, which I've now done.