How do I stop iTunes from randomly removing songs from my phone library?

Did you recently stop subscribing to the Apple Music subscription service or to iTunes Match?

If so, might this apply?

If songs are missing from your music library after you turn on Sync Library - Apple Support

"To use Sync Library, you need a subscription to Apple Music. You can also use Sync Library with an iTunes Match subscription.

If you canceled your subscription to Apple Music or iTunes Match, your music library is removed on all of your devices except for the device your music library is stored on. Any music, including playlists, that you added or downloaded from the Apple Music catalog is also removed."