Thanks for the response. Unfortunately refreshing didn’t work, but I have managed to fix the problem – for reference this is what I did:
I transferred some of the Artist Image files over the Album Artwork folder, and they showed up perfectly, so I looked at the source of the JPGs. All my Album Artist files are downloaded from whatever source I can find, but the Album Artwork JPGs are generated using an AppleScript that takes album cover art from my iTunes Music Library and copies it as a JPG to my Album Artwork folder. I noticed that all the Album Artwork JPGs are exactly the same file size – 273KB; they’re also standardised to 300×300 pixels, so I wondered if the JPG format generated by the AppleScript had some non-standard attributes (because it’s Apple…).
I ran the contents of my Album Artwork folder through Pixillion Plus (Image File Converter). The input and output files are both JPGs, but the output files have a smaller file size, although both versions are 300×300 (Example – input JPG 273KB, output JPG 100KB. All the Input files, as stated above, were 273KB, but the converted output JPGs range from 5KB (Beatles White Album cover, so not surprising up to to around 400KB.
So my assumption is that either Apple Music or the AppleScript itself adds a stack of padding and/or metadata that F-Stop can’t process – basics such as Artist, Album, and Image Size are read, but the image itself gets blocked. Pixillion recognises it as a JPG but appears to strip out all the non-standard content.