This is just another manifestation of our old favourite bug which
is documented under "Known issues and warnings" in http://
lilypond.org/doc/v2.13/Documentation/notation/special-rhythmic-
concerns#grace-notes.
However, your example is a bit more subtle, since you do property
settings that happen in parallel with the grace notes and it seems
that LilyPond thinks that these settings happen
after the grace notes, which means that they have not taken effect
when the StaffGroup is created.
There are two simple remedies:
1) Move the settings to the \with{...} clause:
\score {
\new StaffGroup = "Guitar"
\with {
\consists "Instrument_name_engraver"
instrumentName = #"Guitar"
systemStartDelimiter = #'SystemStartBrace
}
<<
\new Staff {
\initialAcciaccatura
}
\new TabStaff {
\initialAcciaccatura
}
>>
}
2) Move the settings to within one of the Staff contexts that you
define and do them before the grace notes:
\score {
\new StaffGroup = "Guitar"
\with {\consists "Instrument_name_engraver"}
<<
\new Staff {
\set StaffGroup.instrumentName = #"Guitar"
\set StaffGroup.systemStartDelimiter = #'SystemStartBrace
\initialGrace
}
\new TabStaff {
\initialGrace
}
>>
}
Of course, alternative 1) is gives a more aesthetically pleasing
input file.
/Mats