Guess Case behaves differently, depending on whether you apply it to tracklists or to recordings/individual tracks. As this is basically the same data, G.C. should behave the same IMHO -> bug.
In my example "Comin' In on a Wing and a Prayer" [1] the tracklist editor considers In to be right, whereas the recording editor makes it lowercase (in). In this case, capitalized In would be right according to SG.

[1] http://musicbrainz.org/recording/140c0cfc-b6e0-4745-986d-2bc7643e6c74