Unable to play downloaded songs offline on iPhone 14 Plus

I have been buying music from Apple for years so have a considerable collection of music from which I like to create playlists. Yesterday, while sat on a plane, I created a new playlist from music that was already downloaded on my iPhone. Once created it wouldn’t let me play it because I was offline. I was able to play all that music as individual albums so I know there was no issue with the songs.

Today sat at home I have tried all my playlists and non of them work while offline even though every song is downloaded on my phone. Why is this? Is there a way to get around this issue?

[Re-Titled by Moderator]

Original Title: Playing playlists offline (not Apple Music)