\lyricsto ruins sequential lyrics

[Dan Eble]

%% It is puzzling that none of cases 2-4 produces the same output as
%% case 1.  I could not find any known issue in the NR that would
%% explain this.
%%
%% Case 2 is extra puzzling because Lilypond warns that the syllables
%% in { Cc2 Dd } are simultaneous.
\version "2.18.0"

music = \relative c'' { c2 c c c }

<<
  \context Voice = "1" \music
  {
    \new Lyrics \lyricmode { Aa2 Bb }
    \new Lyrics \lyricmode { Cc2 Dd }
  }
>>

<<
  \context Voice = "2" \music
  {
    \new Lyrics \lyricsto "2" { Aa Bb }
    \new Lyrics \lyricmode { Cc2 Dd }
  }
>>

<<
  \context Voice = "3" \music
  {
    \new Lyrics \lyricmode { Aa2 Bb }
    \new Lyrics \lyricsto "3" { Cc Dd }
  }
>>

<<
  \context Voice = "4" \music
  {
    \new Lyrics \lyricsto "4" { Aa Bb }
    \new Lyrics \lyricsto "4" { Cc Dd }
  }
>>