Strange Change Staff behavior

[Alberto Simões]

Hello.

I am having a strange behavior with Change Staff. It is not easy to
explain as it is quite strange. Basically, I am having a voice defined
to be drawn on the lower staff, and it is appearing in the upper staff.

As it is not easy to explain, I am sending the lilypond file in attach.
What is happening: lowerB voice is being drawn in the upper staff.

Thanks for any help
Alberto
-- 
Alberto Simões - Departamento de Informática - Universidade do Minho
                 Campus de Gualtar - 4710-057 Braga - Portugal

\version "2.12.2"

\header {
  title = "Cristo Ressuscitou"
  composer = "J. dos Santos"
  poet = "P. Minhava"
  tagline = "Transcrita por Alberto Simões"

  ocasioes = "entrada"
  seccao   = "entrada"
}


global = {
  \key ees \major
  \time 2/4
}

melody = \relative c' {
  \clef treble
  \global
  \autoBeamOff
  r4\f ees8^\markup{Coro} g \bar "|:"
  bes bes g bes
  ees4( c)
  bes2
  c4 f,(
  \break

  f8) g aes f
  bes4 ees8 c
  aes aes c aes
  f2
  ees2 ~ 
  ees8 \fermata d^\p d d

  g4 f8 f
  g g d d
  g g a a
  g2 ~
  g8 a bes c
  d4 ees8 c

  d8 g, a bes
  c d ees c
  d4 bes8 bes
  g g f f
  g2 ~
  g8 r ees g \bar ":|"
}

text = \lyricmode {
  A- le- lu- ia! A- le- lu- ia!
  Cris- to res- sus- ci- tou.
  A- le- lu- ia! A- le- lu- ia!

  Não pro- cu- reis en- tre_as som- bras a pes- so- a de Je- sus.
  Res- sus- ci- tou: e- ra_a Vi- da.
  Bri- lhe_en- tre vós: é a Luz.
  Bri- lhe_en- tre vós: é a Luz.
  A- le- 
}

upperA = \relative c' {
  r4 ees8[( g]\bar "|:" <g bes>4) g8[( bes]
  <bes ees>4) c8[( ees] 
  <c g'>4) ees8[( g]
  c[ bes aes g]
  f[ ees d c]
  bes4 ees8[( c]
  aes4) c8([ aes]
  f2
  ees2 ~
  ees8) \fermata

  d8([ d d] 
  g4 f
  g d
  g a
  g2 ~
  g8) a[( bes c]
  <f, a d>4) <ees g c>(
  <f a d>) <ees g c>(
  <f a d>) <ees g c>(
  <f a d>) <ges bes ees>(
  <g bes e>) <aes d f>(
  <g d' g>2) ~
  <g d' g>8 r ees[ g] \bar ":|"
}

upperB = \relative c {
  \change Staff = "lower" ees4(--  
  \change Staff = "upper" ees'4)
  <d f>4 g
  <c, g> c'
  <g bes> ees'
  <f, c' d>-- r4

  <bes, f' g>-- r4
  <ees f>-- r
  <c ees>-- r
  d2
  bes ~
  bes8 r c4(

  bes) c(
  b) c(
  bes) c(
  b) c(
  bes8) r8 r4 s2

  s2 s2 s2 s2 s2 s4 ees
}

upper = {
  \global
  \clef treble
  << \upperA \\ \upperB >>
}

lowerA = \relative c' {
  s2 \bar "|:" s2 s2 s2 s2
  s2 s2 s2 bes4 c8[( aes] g2) ~ g8 r r4
  r8 d8[ d d] g4 f8[ f] g[ g] d[ d] g[ g] a[ a] g r8 r4 
}

lowerB = \relative c' {
  s2 \bar "|:" s2 s2 s2 s2  
  s2 bes2 ~ bes ~ bes4 bes,( ees) bes8[( g] ees)_\fermata s8 s4
  g2 ~ g2 ~ g2 ~ g2 ~ g8 s8 s4 c'2 ~
  c ~ c ~ c ~ c ~ c ~ c8 r8 s4
}

lower = {
  \global
  \clef bass
  << \lowerA \\ \lowerB >>
}

        
\score {
  <<
    \new Voice = "mel" { \autoBeamOff \melody }
    \new Lyrics \lyricsto mel \text
    \new PianoStaff <<
      \new Staff = "upper" \upper
      \new Staff = "lower" \lower
    >>
  >>
  \layout {
    \context { \RemoveEmptyStaffContext }
  }
  \midi {
    \context {
      \Score
      tempoWholesPerMinute = #(ly:make-moment 63 4)
    }
  }
}