Unfortunately, this didn't work. The problem is intermittent and didn't happen for a few days, but it's back.
This happens most often when I'm commuting and have no access to wi-fi or cell reception. I tried turning off wi-fi and bluetooth anyway to see if it would fix the problem, but it didn't.
In addition, when I turn the iPhone screen off and go back to music later, it will often take me back to the main page. For example, if I'm listening to a particular release, when I turn off the screen, I'll see the song titles. When I go back to the music app later, sometimes it takes me back to the main screen where I have to reselect 'downloaded music' and then 'artist'. Not sure if that's related or not.