Per http://musicbrainz.org/doc/Picard_Scripting the copy function is supposed to "cop[y] multi-value variables without flattening them."
This was accidentally broken in https://github.com/voiceinsideyou/picard/commit/a6f46df097f935feb7d87b09d58ff8ef79fb576b it seems.
Merged by Lukas a few days ago.
Thanks - I hadn't seen that he'd merged this.
See http://forums.musicbrainz.org/viewtopic.php?id=703 for another issue caused by this - getting stuck in options page unable to validate file naming script.
Confirmed as fixed in current trunk/main/next release.