Chord names collide with notes (vertical space missing on second line staff)

[Jean-Alexis Montignies]

>
% no problem without mark alignment to the left or lyrics
% third staff line is to show that there's no problem after second line

\score {
    <<
        \new ChordNames = "chords"  \chordmode { \repeat unfold 6 {g1 c:m } }
       \new Voice = "theme"{ 
         \repeat unfold 3 {
         \once \override Score.RehearsalMark #'self-alignment-X = #LEFT
         \mark "mark"
         \repeat unfold 2 {g''4 b'8 g'  c'' b' g'4 |  d''8 g' c'' g' b'  a' r4 }
         }
       }
       \new Lyrics \lyricsto theme \lyricmode { \repeat unfold 72 "ba" }
>>
}