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

Finger Under Bracket.ly
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Finger Under Bracket.pdf
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