Hi there, first of all I have to say that mp3tag is great software. I've used it to organise my entire FLAC collection and it's made the job so much easier.
There's an issue I've noticed with the latest version of mp3tag - when I edit the _FILENAME field within my FLAC files (usually with Actions) it adds a field to my FLAC files, I can see this in "Extended Tags"
e.g. with multiple files selected I see:
I've tried leaving the field in and closing mp3tag, I can still see this field from inside foobar so I have to remove the field myself.
I noticed this happened in previous versions of mp3tag but it was automatically removed when I would close mp3tag.