I downloaded some music tracks from the iTunes Music Store and synced them to my iPhone. While browsing the music on my iPhone, I realized the "Composer" field was filled in with the names of the musicians who wrote the songs. However, I only use the "Composer" field for classical music, and leave it blank for non-classical music, so I went back to iTunes and deleted the contents of the "Composer" field for each track, and re-synced. iTunes showed that the files were being updated, but when I looked on my iPhone, it still had entries in the Music app's "Composer" tab. So I went back to iTunes again, and changed the "Composer" field for each track to "Blah", then re-synced. As expected, there was now only a "Blah" composer entry in the Music app. I then deleted the contents of the "Composer" field again (in iTunes) and re-synced. Lo and behold, there was still a "Blah" composer entry in the Music app! So then I unsynced all my music from my iPhone, and re-synced it—and there's still a "Blah" entry!
What do I have to do to get the Music app to recognize that the tracks don't have a "Composer" field anymore?! (iTunes must be one of the most frustrating pieces of software Apple has ever made.)