... until I go to Settings > Library > and hit "Music Folders". I don't have to change anything. I immediately back off of that menu and the files become visible to the app again!
Other things I tried but which didn't change anything: regranting permission to the SD card, granting permissions to my SD Card/Music folder instead of the root, rescanning or full rescanning.
Nothing besides accessing the "Music Folders" selection interface, and ONLY accessing it (no need to change any