So this happened to me.
I have quite a few playlists on Amazon Music.
However after an app update on my iPhone ALL of my playlists were empty. They were still listed but no songs were displayed in them. (I checked my desktop app, and they playlists were still full of songs)
After searching for a solution. I found none, so by trial and error I found the solution to fix amazon music missing playlists.
Here is the solution for IOS.
Open Amazon Music App, Click the three dots in the upper right corner.
Scroll Down to “Settings”
Then try clicking “refresh my music” this got some of my music back, you may have better luck.
If that doesn’t get all of your music back, then on the same screen, click “Sign out” And then sign back in to the app. Voila, all of my playlists were full of songs again.
NOTE: this will remove all of your downloaded music and you will have to re-download it. However it brought all of my playlists back.