What causes Apple Music to unload songs on iPhone, and how can I recover them?

I have loaded many albums of music on my IPhone 16 pro with the latest version of IOS. I don’t listen to the music too often but usually when I travel I listen quite a lot. On a recent trip (May to mid June) I tried to listen to some of my albums but while the album is still present, only one track was available. This was true on several of the albums. For the playlists that I created from combining other albums, those were intact. What’s going on with my music? Please help.

[Re-Titled by Moderator]

Original Title: why is my apple music being unloaded from my iphone