Audiobooks only visible in FOLDER mode, not in BOOKS mode?

I’ve noticed many of my audiobook folders don’t appear at all in the Books > Audiobook mode. I’m using MP3Tag and ID3v2.3 ISO 8859-1, but have tried v2.4, as well. I’m using firmware 1.29. 

I’ve removed the tags down to the bare bones with just Title (e.g. 01.mp3) and Album (e.g. ABC), but still, about half or two-thirds of my folders are only visible in Folder mode.

Thanks to Miikerman’s suggestion about FAT Sorter, I’m finally able to display and play my tracks in sequential order. Alas, I’m only able to view the albums/folder in FOLDER mode

I see other users have had similar issues, but it doesn’t look like they found a resolution…