What happens if you take a track that you haven't added to Airtime yet, modify it in the way you described above, and then add it to Airtime. Does Airtime see those metatags?
just to check since I read your other post, this problem is in version 1.8.2? Airtime 1.9+ has a totally different way of getting file metadata so it would be good to test importing these files there when you upgrade versions.