String Number Appears, Then Disappears

[Mark Mathias]

Greetings!

Why does a String Number appear under a Horizontal Bracket in the first
instance, but disappear in the following score?

And a tangential question: why don't certain engravers, such as
Horizontal_bracket_engraver, have a context?

Cf.,
http://lilypond.org/doc/v2.24/Documentation/internals/horizontal_005fbracket_005fengraver

Thank You in Advance!
Mark Mathias

\version "2.24.0"

% Why don't certain engravers, such as Horizontal_bracket_engraver,
% have a context?
% Cf., http://lilypond.org/doc/v2.24/Documentation/internals/horizontal_005fbracket_005fengraver

\layout {
  \context {
    \Voice
    \consists Horizontal_bracket_engraver
  }
}

% Why do String Numbers appear under a Horizontal Bracket here but
% disappear in the following score?

\score {
  \relative c'' {
    \override HorizontalBracket.outside-staff-priority = 2000
    \override HorizontalBracket.direction = #UP
    \time 6/8
    c8\1 \startGroup c^2 c^\markup \circle \finger "3" c^2 \stopGroup c c
  }
}

% Why do String Numbers disappear under a Horizontal Bracket here but
% appear in the first score?

mel = {
  \time 6/8
  c8 c c c c c
}

mldFNb = {
  \time 6/8
  \override HorizontalBracket.outside-staff-priority = 2000
  \override HorizontalBracket.direction = #UP
    s8\1 \startGroup s^2 s^\markup \circle \finger "3" s^2 \stopGroup s s
}

staffMelB = \new Staff {
  \relative c'' { \new Voice
    <<
      \mel
      \mldFNb
    >>
  }
}

\score {
    \staffMelB
}

Finger Under Bracket.pdf
Description: Adobe PDF document