TGK, is your problem different as the case from user david.lynch?
see here:
[WS] Discogs (pone mod)
Look at the UNSYNCEDLYRICS field in the screenshot. Squares are displayed in the extended tags dialog, but not in the "edit tag info" window for the single field.
He writes that those squares are only displayed in some views in Mp3tag, but acctually they are not there. Also they don't show up when the files are displayed in his media player.
here is a older topic about a similar problem, not related to tag sources, but maybe it helps you:
/t/6177/1