Music purchased disappeared

First check they are not hidden 

Hide and unhide Music ...

https://support.apple.com/en-ca/HT208167

Redownload apps, music, movies, TV shows, and books from the App Store, iTunes Store, and Apple Book

https://support.apple.com/en-ca/HT201272

On an iPhone, iPad, or iPod touch

1. Open the iTunes Store app.
2. On your iPhone or iPod touch, tap  More in the menu at the bottom of the screen, then tap  Purchased. On an iPad, just tap  Purchased. 
3. Depending on what you want to redownload, tap Music, Movies, or TV Shows. If you use Family Sharing, tap My Purchases or choose a family member’s name to see content that they purchased.
4. Find the movie, TV show, or music that you want to redownload, then tap it. 
5. Tap  to the right of the title. The movie, TV show, or music downloads to your device. You can find movies and TV shows that you redownload in the TV app§ or Videos app, and music in the Music app. Don't see the item that you purchased?

The exact same thing happened to me, and I’ve had that album since 2015. I’ve looked in my hidden purchases and my purchased history. What is happening