I just realized that Mezzmo will allow you to attempt to change the tag fields by right-clicking on a file or group of files and then selecting properties.

I say attempt because some of these updates appear to complete with no errors (eg ARTIST field in Mezzmo displays the updated value), but then when I trigger a "Maintain Playlist" the update disappears (eg ARTIST field in Mezzmo then displays blank or previous field)?

Any ideas why some of the updates aren't sticking?