polyphony and grace notes breaks beaming

[Magnus Lewis-Smith]

Example lily source as follows.  All beaming after the second bar is broken.

I assume this is another variant of the  grace note bug 
http://lists.gnu.org/archive/html/bug-lilypond/2005-07/msg00050.html

\version "2.6.5"
\relative c' {
  \time 2/4
%% OK HERE
  \appoggiatura { c16[ c] }
  c8 c c c |
%%% BROKEN NOW
  << {
    \grace { s16 s } \voiceOne
    g='8 g g g
  } \\ {
    \appoggiatura { e16[ e] } \voiceTwo
    e8 e e e
  } >> |
%%% STILL BROKEN
  \appoggiatura { c16[ c] }
  c8 c c c
}


Magnus