Can't play playlist of owned music. "connect to wi-fi or mobile network" message

First off, I don't subscribe to Apple Music. I don't do streaming.

This issue concerns owned music, ripped from CDs.

Most of the music on my iPhone is ripped from CDs I own, with only about 6 or so albums being paid digital downloads from iTunes.

Recently I made a new playlist with music downloaded to my iPhone from my MacBook that is all from CDs I own.

Today I was out shopping and wanted to listen to music, so I decided to listen to a playlist I'd recently made using music that is from CDs I own.

Every song in the playlist is from an album that is fully downloaded to my iPhone.

However, this particular playlist kept telling me to "connect to wi-fi or mobile network to listen to music".

Being out I am obviously on my mobile network, and I don't connect to public wi-fi for safety reasons.

I tried other playlists, and even the albums and the individual songs that are in this particular playlist, and they all played fine without being connected to wi-fi.

When I got home and connected to my home wi-fi the playlist in question played fine.

I must stress again, I do not have an Apple Music subscription. All the songs on my iPhone are either ripped from CDs I own or are digital downloads from iTunes.

So why does this one playlist refuse to play unless I am on wi-fi?