I have a particular multiartist album that Picard acknowledges as multiartist when it comes to setting metadata, but it's acting as if it's not multiartist when it comes to naming the files. (For comparison, I have a very similar multiartist album that works just fine, as do all my other ones.)
My filenaming code:
The relevant bit being at the very end. Changing $if(%_multiartist%,%artist% - ,) to $if($or(%_multiartist%,$eq(%genre%,Electronic)),%artist% - ,) works as a temporary workaround, so it looks like the file naming script thinks %_multiartist% = 0 for this release, even though the other scripts think it's 1.