I have found this problem to exist in all beta versions including the release candidate. I use the following string:
CODE
$if(%album artist%,'various',%artist%)\%album%\$num(%tracknumber%,2)$if(%album artist%,'. '%artist%' - ','. ')%title%
as the filename in the converter. I also put this in my playlist. The idea is that albums with the album artist tag in them will get written to a 'various' directory and that the individual track have the track artist in their filename (provided that the 'album artist' tag reads 'various'). All other songs are written into a 'artist/album/##. title' format.
This has always worked flawlessly in 0.8. However, in 0.9 foobar treats ALL tracks as if they had an 'album artist' tag present.
See the below screenshot, the 'moody blues' cd does not have an album artist tag but still it is treated as such in my string.
Screenshot
Ganymed: linked to big screenshot