Overriding position of a full bar rest does not always work

[Peter Toye]

The following MWE shows that the full bar rests are not shifted as I would have expected in the parallel voices. Bars 5 and 6 print the rests at different heights. Have I missed a property somewhere? I experimented with voiced-position but that didn't affect the full-bar rests.

Alternatively, can some one help me place the markup at the beginning of the bar? Seems eccentric here, but necessary in a multi-staff score.

 
Regards,

Peter
mailto:address@hidden
www.ptoye.com

\version "2.19.52"

\language "english"

\score {
  \new Staff {
    \override Score.BarNumber.break-visibility = ##(#t #t #t)
    c''2 r2 d''2
    \override Rest.Y-offset = #2 r2
    c''2 r2 R1
    <<
      {
        s1^\markup { \italic "Ritard." } s1 s1 s1
      }
      \\
      { \override Rest.Y-offset = #1  R1 r1
        \revert Rest.Y-offset R1 r1      
      }
    >>
    R1
  }
}