I duplicated the same problem in QCD. So, I wrote the tag using Mp3Tag. Re-opened the playlist and still had the same behaviour . . . also with the tag showing blank. I then deleted that playlist, and created a new one with the same files.
Now, when I go to that playlist, I can open the same song's tag, and edit it.
It almost appears as though the first playlist somehow was interfering with the ability to read, or write to the file.
Anybody ever seen anything like this before? BTW - I opened the playlist in NotePad and it appeared normal. Weird!
Artie