Today I tried to add an "a cappella" attribute to the recording-work "performance of" relationship (https://musicbrainz.org/relationship/a3005666-a872-32c3-ad06-98af558e99b0) but that immediately broke phrase interpolation in most places (although not all, surprisingly the examples showed correctly in the edit relationship type page).

The only way I found of getting it working again was to rename the attribute to "acappella" (one word) and then using {acappella:a cappella} on the phrase. While this works, it's not ideal, and it was just a coincidence we only hit this now - we've had multi-word attributes for a long time, just not used them on interpolation. We should add support for interpolating multi-word attributes, and then rename "acappella" back to "a cappella".