Just an update on my specific case and I think I’ve solved the issue I was having with the iPhone music app just stopping.
the issue appeared to be with the playlists and music files on the phone. I sync up via pc iTunes over cable and push the data from the pc to the phone generally but a sync pulls data as well.
I suspect my music app data was just corrupted after 6 years of use without a new sync.
to fix my issue I deleted the playlists from my tunes , synced with the phone, then recreated new playlists on the PC and synced again. This way the music files and the playlists has to be recreated on the phone.
after That sync all seems to be well.
so if the iPhone music app is unexpected stopping playback - it may be due to corrupted playlist data or music file system on the phone. Wipe and resync it all.