I am not sure if this is a problem with MP3Tag but maybe someone here has a good idea as to what is happening:
I use a combination of MP3Tag, MusicMagicMixer and Slimserver. Both MMM and MP3Tag allow me to change tags. I use MP3tag for most of my tagging (a brilliant progam!) but every now and then use MMM if I encounter some wrong information there. The problem is that if I change a tag in MMM, it is seen OK by Slimserver but in MP3Tag this changed tag shows up as a blank (empty) field. Also in Foobar the changed tag shows up correctly. If I change a tag in MP3tag it is seen OK by both Slimserver and MMM.
Although this is easy to work around (just use MP3TAG) I wonder what could cause this; why is the same tag seen be one program and not seen by the other.