Strange behavior with grace notes

[John Burt]

When the first notes of a piece are grace notes, if there are other voices,
or other staves in the piece, the grace note is set in a separate bar, and
then the time signature and key are printed again. I notice the same thing
with appoggiatura and accacciatura. I tried to make this example as minimal
as i could.

\version "2.18.2"
global = {
  \key f \minor
  \time 3/4
}

tenorI = \relative c' {
  \global
  c2.
}

tenorII = \relative c' {
  \global
   \grace {c16  des16} c4 des8 des8 c8
}

baritone = \relative c {
   \global
  \dynamicUp
  c2.
}

\score {
  \new ChoirStaff <<
    \new Staff \with {\clef "treble_8"
    } <<
      \new Voice = "tenorI" { \voiceOne \tenorI }
      \new Voice = "tenorII" { \voiceTwo \tenorII }
    >>

    \new Staff \with {
    } <<
      \clef bass
      \new Voice = "baritone" { \voiceOne \baritone }

    >>
  >>
  \layout { }
}

if I comment out the tenorI voice and the baritone voice the grace note
works fine. But if I uncomment either the problem returns.

Any light anyone can cast on this would be much appreciated!
John