Double-stemming question.

[Hwaen Ch'uqi]

Greetings,

In the following measure of code, there are several instances of
double-stemming. The single notes and triads merge correctly, but the
four-note chords do not. I feel like the reason and solution should be
straightforward, but I cannot seem to wrap my head around the
situation. Any help would be greatly appreciated. Please forgive the
slight length of the code. I have also attached the pdf output.

Hwaen Ch'uqi

\version "2.20.0"

\score {
  \new PianoStaff <<
    \new Staff = up {
      \relative c' {
        \key gis \minor \time 4/4 \clef treble
        <<
          {
            \mergeDifferentlyHeadedOn
            s8.. <cis'' eis gis>32 <cis eis gis>4 s8.. <fisis ais cis dis>32
<fisis ais cis dis>4
          }
          \\
          {
            s1
          }
        >>
      }
    }
    \new Staff = down {
      \relative c' {
        \key gis \minor \time 4/4 \clef bass
        <<
          {
            \mergeDifferentlyHeadedOn
            \mergeDifferentlyDottedOn
            gis,,32 gis'' cis \change Staff = up \voiceTwo eis eis' cis gis'
<cis eis gis> <cis eis gis> gis cis eis, gis, \change Staff = down
\voiceOne eis gis, gis,, gis gis'' dis' \change Staff = up \voiceTwo
fisis fisis' cis ais' <fisis' ais cis dis> <fisis ais cis dis> ais,
cis dis, ais \change Staff = down \voiceOne fisis gis, gis,,
          }
          \\
          {
            gis4... gis32 gis4... gis32
          }
        >>
      }
    }
  >>
}

MWE.pdf
Description: Adobe PDF document