Yeah, I was wondering if it might be a scoped storage issue -- but I thought that wouldn't affect something like a music player. With scoped storage, I think the music player would have to create its own music directory in order to display those files. The odd thing is that Music Player (Muzio) could still display music tracks in the Folder section -- just not in the main sections where it would use the embedded ID3 tags of the music files.
In addition, when I tested that app, I noticed a directory called /AudioBeats (which was another previous name for the app) with a few subfolders related to playlists. I tried adding an mp3 file there, but it still wouldn't show up. Since /AudioBeats was clearly created by the app, I figured it should've worked, so I wonder if that indicates more of a bug on their side.