Downloaded songs disappearing from Apple Music playlists

Are you saying the Album is no longer available in Apple Music? The Artist/Publisher controls what music is made available on Apple Music. They can add music and pull music from the service at any time. It works the same way for Spotify and Pandora. Apple is not determining what tracks should be available and even you could create a song and add it to Apple Music through a Publisher where it will remain until you remove it. You are free to choose any music service you wish.

Downloading songs from Apple Music is not the same as purchasing them from iTunes. A song purchased from iTunes will remain after the Artist/Publisher removes it from the service and you can continue to listen to it. None of the streaming services will allow you to continue to play a song that has been removed from the platform.

.... often lauded as the world's biggest data base of music.

Not even close. Apple Music recently surpassed 100 million songs in its catalog. Soundcloud has around 375 million.

But more to your point, songs can vanish and reappear in the catalog without warning. The usual cause is some legal dispute among various rights-holders (or claimants). The exact reason in any case is rarely publicized. Many of the disappearances are temporary, but there is no way to know if and when a song might reappear.