Extra bracket in score in Lilypond 2.7.18

[liang seng]

Hi, I'm typing a score layout and here is the file:

\version "2.7.17"
\layout {
 \context {
   \Score
   \accepts "OrchestraStaffGroup"
 }
 \context {
   \StaffGroup
   \name OrchestraStaffGroup
   \remove "Span_bar_engraver"
   \accepts "InstrumentStaffGroup"
 }
 \context {
   \StaffGroup
   \name InstrumentStaffGroup
   \remove "System_start_delimiter_engraver"
 }
} \score {
<< \relative
\new StaffGroup <<
\new Staff << {  R1 } >>
\new Staff << {  R1 } >> >>
\new OrchestraStaffGroup <<
 \new InstrumentStaffGroup <<
      \new GrandStaff <<
     \new Staff { R1 }
     \new Staff { R1}
   >>
  \new Staff << { R1  } >> >>
\new InstrumentStaffGroup <<
   \new Staff <<
     \context Voice = melody { c'1 }
     \lyricsto melody \new Lyrics { do }
   >>
 >>
 \new InstrumentStaffGroup <<
   \new Staff { \clef bass R1 }
   >> >> >> }

Here, the bar spanning and bracketing is intended as so. But what happened 
is that there is an extra bracket before the 3rd to the 5th staff (from the 
top), which is between the brace on the left and the longer bracket at the 
right. Why did this bracket appear? I explicitly put \remove 
"System_start_delimiter_engraver" because I would like to remove the extra 
brackets. This problem did seems to be caused by the new system delimiter 
feature in 2.7.18 as in versions 2.7.17 and before, the problem didn't occur 
at all. What can be done?
Thank you.