I just tested it - and can partially confirm the bug (which looks like a variation of the 2.78a bug described here [F] 2.78a: Konverter Tag-Dateiname mit \ führt zu falschem Pfad
The problem is the combination of path and variable with slash in the data being part of the path (and not just the filename).
If you leave out the variable in the path then MP3tag behaves as it should: no path repetition.
Also, if you treat the whole format string with a $validate()-function then the path comes out correct as well.
But I agree: it looks like a bug.