iPhone 7 Stops Playing "Apple Music" after 2 Hours Uninterrupted

I have an iPhone 7, running iOS 15.8.5. I have several films downloaded to it, all of which are tagged as “music videos” and playing randomly in an Apple Music playlist. I'm not doing anything else on it. After 2 hours, it will stop playing.

A few more details…

-It's in Airplane mode.

-It’s plugged into the wall.

-I’m not casting this to another screen. Just playing the iPhone.

-I had a similar setup with the same iPhone 7, only it was running 12.4.1 and it was a playlist in iTunes—and it never did this, always played endlessly without a hiccup.

Are there any default settings that happened when I went to 15.8.5 that could be doing this?

Thanks!