Apple Music has become increasingly hostile to Apple Music library files. What I mean is audio files you create and want to store in Apple Music. Even the interface to manually manage songs is grayed out and doesn’t show you your files that are stored locally in the Finder. In addition, unrealBook only supports locally stored audio files in the iTunes/iPad/iPod library. It always only supported local files, but at some point Apple did or did not support linked cloud files. On iOS12 it certainly doesn’t. On iOS15 it seems like it actually does supported cloud based songs.
For this reason, (years ago) I implemented direct linking of supported audio files (mp3, mp4, wav, aif etc) in the unrealBook documents folder. It’s easy to use. Simply Airdrop, dropbox, open in…. any audio file to unrealBook and it will be imported. After that, use the CD Menu->Music Controller ->link audio file button or CD->menu->play audio file->then + button or create a transmogrify play button and use the use file button to link the audio file.
This is way more reliable, has none of the problems of linked Apple Music and is not subject to any iOS changes to Apple Music.
In short, don’t use Apple Music for linked audio files in unrealBook if you can.