There is a bug relating to the export of tags into text files.
The bug only seems to occur on albums with 8 or less mp3 files. Exporting 9 or more doesn't produce this strange behaviour.
This is what happens: The first track gets appended to the bottom of the list, so when you import the file back as tags, well, you know know what happens.
Rammstein - Das Modell - Kokain - 2/3 - 1997 - Metal -
Rammstein - Das Modell - Alter Mann [Special Version] - 3/3 - 1997 - Metal -
Rammstein - Das Modell - Das Modell - 1/3 - 1997 - Metal -
Hope this helps. Mp3tag is still the best tagger on earth.