Why does iTunes/Apple Music remove albums from time to time?

For some reason, every time I go to listen to certain albums such as Mechanize by Fear Factory, Demigod by Behemoth, The Price of Existence by All Shall Perish, Shot to **** by Black Label Society, nearly every album by Decapitated, and many more, they look like they’ve been removed from the library. Not just my library, but on Apple Music and iTunes as well. Sometimes these albums will be put back up, other times different albums get taken down. Spotify, Deezer, Google Play, and other streaming services seem to have this same problem. Does anyone have any idea what this is and why it keeps happening?