Why does iOS keep deleting some of my music and switching to cloud version?

[Engram]

Noticed this problem on iOS7 and still have it on iOS8.

I purchase an album, and sync the entire thing to my iPhone 5S 16GB with iTunes. A few days later I go to play the album and notice that a few tracks are showing the "download from cloud" icon, indicating that they are not being stored locally.

Why?!

Does iOS auto-delete music if you run low on space? I'm not using iTunes Match or Genius. I have iTunes configured to sync "All music", not by playlist.

 

[Engram]

At least I'm not the only one! It's bizarre that I can't seem to find anyone else discussing this though. Doesn't help that there's loads of hits for the U2 album fiasco, if you start Googling "ios cloud music" etc.

P.S. As annoying as this is, it's not enough to send me to Android.