autoChange creating extra staff

[Alexandre Loomis]

Hi, I'm running into an issue using \autoChange. I'd like CrossStaff to
share the lower staff (and upper with another voice, but that doesn't seem
to cause any issues), but it's creating its own.

\version "2.24.0"

CrossStaff = \autoChange f {
  \relative c' {
    c2 c,
  }
}

VoiceTwo = \relative { \clef "bass" c,1 }

\new PianoStaff <<
  \new Staff = "up" {
    \new Voice \CrossStaff
  }
  \new Staff = "down" {
    \new Voice \VoiceTwo
  }
>>