Finally, someone else who feels the same pain as I do. Believe me you, I keep subscriptions of both Apple Music and Spotify. Apple Music for the better in general catalog and better syncing, Spotify for better discovery. I plan on creating a tool that would allow me to download all the songs in my library so that I don't lose on them when Spotify or AM decide to remove it from their catalog. Apple Music, at least, shows the track details but greys it out. Spotify simply shows an empty entry, which frustrates me even more.

There’s an app called song shift that works 90% well. It’ll transfer the music over from one service to another. Also you can export Apple songs to CSV using desktop iTunes

That's solving a different problem. If you have a track that is not on any service, Apple Music will still accept the song file being added to your library and sync it for you. Spotify will not as it does not do cloud file sync.