unfoldrepeats makes midi tracks follow one another

[John Comeau]

I'm probably doing something wrong, but have tried several
permutations of wrapping things inside one another with the same
result: the tracks follow one another in the MIDI output though the
graphical representation is correct. Googling and searching archives
for unfoldrepeats turns up nothing of value that I could see.

downtown_sandiego.ly:

tromboneNotes = \relative c {
 c16 c c8-. r c16 c d d d8-. r d16 d e e e8-. r e16 e f f f8-. r f16 f
 e16 e e8-. r e16 e d d d8-. r d16 d e e e8-. d16 d d8-. e16 e e8-. d16 d d8-.
}
trombone = {
 \time 4/4
 \set Staff.instrument = #"Trombone"
 \set Staff.midiInstrument = "trombone"
 \clef bass
 \repeat volta 2 {
  \context Staff <<
   \tromboneNotes
  >>
 }
}
drumNotes = \drummode {
 hh8 sn hh sn hh sn hh sn hh8 sn hh sn hh sn hh sn
 hh8 sn hh sn hh sn hh sn hh8 sn hh sn hh sn hh sn
}
drumContents = {
 \set DrumStaff.instrument = #"Drums"
 \repeat volta 2 {
  \time 4/4 <<
   \new DrumVoice { \voiceOne \drumNotes }
  >>
 }
}
\score {
 <<
  \context Staff = trombone \trombone
  \new DrumStaff { \drumContents }
 >>
 \layout { }
}
\score {
 <<
  \unfoldrepeats {
   \new DrumStaff { \drumContents }
   \context Staff = trombone \trombone
  }
 >>
 \midi { \tempo 4=120 }
}

Dump of downtown_sandiego.midi (http://jcomeau.com/src/midi/midi.py,
symlink to mididump)

[ # midi file dump
[ # dump midi header
'MThd',
6, # chunk length
1, # MIDI file format
3, # Tracks
0x180, # Time Division
], # end of midi header dump
[ # midi track 1
'MTrk',
248, # chunk length
[0, 0xff, 0x1, 39, 'Creator: GNU LilyPond 2.4.2            '],
[0, 0xff, 0x1, 58, 'Generated automatically by: GNU LilyPond 2.4.2            ']
,
[0, 0xff, 0x1, 60, 'at Sun Jan  9 16:22:19 2005
'],
[0, 0xff, 0x1, 60, 'at Sun Jan  9 16:22:19 2005
'],
[0, 0xff, 0x3, 7, 'Track 0'],
[0, 0xff, 0x2f, 0, ''],
], # end midi track
[ # midi track 2
'MTrk',
625, # chunk length
[0, 0xc9, 0], # change instrument for channel 9 to Acoustic Grand Piano
[0, 0xff, 0x4, 5, 'drums'],
[0, 0x99, 0x2a, 0x7f], # Note On, "gf,", channel 9, velocity 127
[0, 0xff, 0x58, 4, '\x04\x02\x12\x08'],
[0, 0xff, 0x3, 14, 'uniqueContext2'],
[0, 0xff, 0x51, 3, '\x07\xa1 '],
[192, 0x89, 0x2a, 0x40], # Note Off, "gf,", channel 9, velocity 64
[0, 0x99, 0x26, 0x7f], # Note On, "d,", channel 9, velocity 127
[192, 0x89, 0x26, 0x40], # Note Off, "d,", channel 9, velocity 64
[0, 0x99, 0x2a, 0x7f], # Note On, "gf,", channel 9, velocity 127
[192, 0x89, 0x2a, 0x40], # Note Off, "gf,", channel 9, velocity 64
[0, 0x99, 0x26, 0x7f], # Note On, "d,", channel 9, velocity 127
[192, 0x89, 0x26, 0x40], # Note Off, "d,", channel 9, velocity 64
...[snip]
[0, 0xff, 0x2f, 0, ''],
], # end midi track
[ # midi track 3
'MTrk',
751, # chunk length
#
# *** NOTE DELTA OF NEXT EVENT -- trombone follows drums! ***
#
[12288, 0xc1, 57], # change instrument for channel 1 to Trombone
[0, 0xff, 0x4, 8, 'trombone'],
[0, 0x91, 0x30, 0x7f], # Note On, "c", channel 1, velocity 127
[0, 0xff, 0x58, 4, '\x04\x02\x12\x08'],
[0, 0xff, 0x3, 8, 'trombone'],
[0, 0xff, 0x51, 3, '\x07\xa1 '],
[96, 0x81, 0x30, 0x40], # Note Off, "c", channel 1, velocity 64
[0, 0x91, 0x30, 0x7f], # Note On, "c", channel 1, velocity 127
[96, 0x81, 0x30, 0x40], # Note Off, "c", channel 1, velocity 64
[0, 0x91, 0x30, 0x7f], # Note On, "c", channel 1, velocity 127
[192, 0x81, 0x30, 0x40], # Note Off, "c", channel 1, velocity 64
[192, 0x91, 0x30, 0x7f], # Note On, "c", channel 1, velocity 127
[96, 0x81, 0x30, 0x40], # Note Off, "c", channel 1, velocity 64
...[snip]
[0, 0xff, 0x2f, 0, ''],
], # end midi track
] # end midi file