Once upon a time, Picard had a variable named %compiler% which contained the value from this exact relationship type: https://musicbrainz.org/relationship/2f81887a-8674-4d8b-bd48-8bfd4c6fa332

In v2.6.3 however, this bit of data no longer appears in the list of tags when tagging a file, but if you try to use %compiler% in a script, it does exist so {{ $if2(%compiler%,%other stuff ... }} will always return an empty string, rather than using the next variable. (If %compiler% wasn't defined, it'd use the next variable.)

Anyway, ideally, the fix for this is to populate this tag with the actual value in this relationship type from the MusicBrainz database.