Despite using id3v2 2.3, It appears that: Picard is storing a TSOP field, and storing it blank.
http://wiki.musicbrainz.org/Picard_Tag_Mapping says XSOP is used for v2.3.

Below is the output of id3v2 in the relevant file:

$ id3v2 -l Serenade\ for\ Strings\ in\ E\ minor\,\ Op.\ 20.mp3
id3v1 tag info for Serenade for Strings in E minor, Op. 20.mp3:
Title : Serenade for Strings in E mino Artist: A Far Cry
Album : Year: , Genre: Unknown (255)
Comment: Track: 0
id3v2 tag info for Serenade for Strings in E minor, Op. 20.mp3:
TIT2 (Title/songname/content description): Serenade for Strings in E minor, Op. 20
TPE1 (Lead performer(s)/Soloist(s)): A Far Cry
UFID (Unique file identifier): http://musicbrainz.org, 36 bytes
TXXX (User defined text information): (Acoustid Id): 2fe81e51-ff24-424f-bf87-41cdca0bdab4
TSOP (): frame
TXXX (User defined text information): (MusicBrainz Work Id): af19cd3d-d69d-46bd-80e4-a298f1a7c09e
TCOM (Composer): Sir Edward Elgar
TXXX (User defined text information): (MusicBrainz Artist Id): 2d65c68d-8b0a-4ceb-85f0-c4c0dbbb46ed