strange detached beam issue

[Jin Choi]

I have a section of piano music where two voices start in the treble clef and 
subsequently move down into the bass clef. When the right hand moves down, the 
beam stays in the treble clef unattached to anything:

When I try to isolate this section, the problem does not occur. I am mystified 
as to what is going on. What might be some things I could try?

\version "2.24.1"

top = { \change Staff = "upper" \voiceTwo }
bottom = { \change Staff = "lower" \voiceOne }

upper = \relative c'' {
  \time 2/2
  \key c \major
  \voiceOne
  r8 bes'\mf( a gis a ees d cis |
  d bes a gis a ees d cis |
  d) bes'\dim( a\! gis a ees d cis |
  d[ \bottom bes a gis] a[ ees d)] r8 |
  \top 
  \oneVoice
  r8\p
}

lower = \relative c' {
  \clef bass
  a4 r8 \top bes'( a gis a ees |
  d cis d \bottom \voiceTwo bes a gis a ees |
  d cis d bes' a gis a ees |
  d cis d bes a gis a cis | 
  \oneVoice
  fis,4)
  
}

\score {
  \layout {
    \context {
      \Staff
      \consists Merge_rests_engraver
    }    
    \context {
      \PianoStaff
      \override StaffGrouper.staff-staff-spacing.basic-distance = #11
      \override TextScript.avoid-slur = #'outside
      \override TupletBracket.bracket-visibility = ##f
    }
  }  
  \new PianoStaff {
    <<
    \new Staff = "upper" \upper
    \new Staff = "lower" \lower
    >>
  }
}