Hi, jonaustin.
I'm not sure what's up with the successful but empty return for Bohemian Rhapsody: that seems like a bug. I'll see what we can do about that.
More broadly though, we do a less-than-complete job of matching MusicBrainz recordings to our universe of music. This is because MusicBrainz (obviously) does not have audio that we can analyze and fingerprint, so we can only match on metadata, which leads to some errors and some failures to match anything in cases where the names that MusicBrainz has for an artist or recording aren't quite the same as the ones in our data. This gap is why we have never officially documented support for MusicBrainz recordings as foreign song IDs as part of Rosetta Stone.
As a practical matter, if you are using lastfm scrobble data, you may have better luck using artist names and track titles.
-Eli