My issue is similar not sure if nayone has described it exactly the same way though.
I make a purchase on iTunes (on iPhone 5S) it downloads, i then go to the Music app (iPod) open the Purchased playlist and scroll through looking for the new purchases only to find they are not in the playlist as they used to be.
I tried purchasing music on my Mac to see if it automatically downloads into the Purchased playlist and STILL does not make its way to that playlist.
The only solution (if you want to call it that) is pressing Edit (in the purchased playlist) and manullay adding the song by searching for it which then adds it to the playlist.
So to be clear... the song IS downloading on to the phone, just not into a playlist as it used to in previous versions of iOS
these issues began with iOS 7 i now updated to iOS 7.1 to see if this would be resolved only to fond that it has not. I am not the only one in my circle of friends with this issue all of my friends who spend their money on buying content from the itunes store in their phones have this same issue and find it just as much of a hassle as i do.
Sad to think these phones were derrived from the best music players in the world (iPods) and to see how poorly they serve that purpose today.