Ripped Music Disappears from Music App on iPhone

Hello! Over the past two years or so, I have been noticing that music content that I ripped from CDs long ago (much of my Beatles, Stevie Wonder, and other music) will disappear from the Music app on my iPhone. In addition, playlists that I create (whether they have CD content or not) will also disappear from the Music app from my iPhone. This tends to be quite annoying as I rely on music I've purchased over the years on my phone to listen to music while out and about (AKA I don't stream music from Apple Music or Spotify or the like), and I usually don't realize that the music has disappeared until I'm out and about (AKA not near a computer). I've tried to research this issue already - online, a lot of folks seem to think these issues arise if your "optimize music" selection is turned on in Settings on your iphone. I have checked - there is no function for this under the Music app in Settings on the phone. Can anyone advise why this keeps happening?