Splitting a library

Actually, a filename can be just the name, you can add the extension and it is still a filename, you can partially qualifiy it with only parts of the path or you can fully qualify it by adding the drive (letter) and full path specification. These are the rules of the OS. And MP3tag follows them.
MP3tag is gracious enough to let you add all the components of a fully qualified filename and does not produce an error message if you switch from a simple filename to a partially or fully qualified filename.
The advantage is that you do not have to take apart the fully qualified filename, each time you want to address only the actual filename.
So I would not consider it as some kind of failure but actually a convenience that you do not have to invent the wheel again and again but have a ready-made variable.