Users writing tagger scripts to fill in missing MusicBrainz data run into a confusing problem: $if(%tag%,,...) sees file's existing tags through the MultiMetadataProxy fallback, making it impossible to check "did MusicBrainz actually provide this tag?"

For example, this common pattern doesn't work as expected:

$if(%composer%,,$set(composer,$if2(%lyricist%,%writer%,%arranger%)))

If the file already has a composer tag but MB didn't provide one, %composer% is non-empty (from the file), so the condition never fires.

Related: https://community.metabrainz.org/t/tagger-script-not-executing-automatically/815447