Why isn't VoiceTwo flipping the stems down?

[Kevin Cole]

What did I miss THIS time?

%%%%%%%%%%
\version "2.22.1"
\language "english"

global = {
  \time 4/4
  \key d \major
}

soprano = \relative c'' {
  \global
  \clef treble
  b2( cs2)                             | % 19
  <<
    \voiceOne { d2 e2 }                  %    Stem UP
    \new Voice
    \voiceTwo { b4 a4 b4 cs8[( b8]) }    %    Why isn't this Stem DOWN ???
  >>
  \oneVoice                            | % 20
  d2 cs2                               | % 21
  \bar "|."
}

\score {
  \new Staff = "soprano" \soprano
  \layout { \autoBeamOff }
}
%%%%%%%%%%