Michael, what Tag-Filename does with a partial filepath is undefined behaviour (*), so I don't think one can rightly say that variable results means it is a bug or a feature.
I've found that the path completer to be the Mp3Tag "current directory". "current directory" became blurred when, between your two versions, Mp3tag added support for multiple directories. What it does now is use the /last/ directory from which a file was loaded, whether by Add Direcory, drag-in file or playlist, or whatever.
I know no way of getting Tag-Filename to respect mutiple load directories. If you can avoid making a load from C:\ e.g. by doing each directory in a spearate load, you should get the result you want. I useful debugging aid is Options, General, 'Show current directory in window title'.
- Florian, it would be great if this could be fixed, e.g. by putting a commitment to the current behaviour in the docs file:///C:/Program%20Files/Mp3tag/help/options_converter.html#ttf