I have noticed this a few times before and I think it is a bug. Ok I am running winamp version 2.95 (last one before version 3) and I put a bunch of mp3's into mp3tag and I looked at the comments of the songs and saw nothing. Well I deleted all the fields regardless. I then played the songs in winamp and saw comments in the ID3v2 section. I went back to mp3tag and nothing. I also noticed there was also words in the composer section in winamp and yet not in mp3tag.
Took a screen cap of winamp those 2 fields don't show any content in mp3tag.