This subheading of vocal.itely states:
"By default, grace notes do not get assigned syllables when using
@code{\lyricsto}, but this behavior can be changed:"
The behavior uses \addlyrics, do you agree to instead have it
contain:
<<
\new Voice = melody \relative c' {
f4 \appoggiatura a32 b4
\grace { f16[ a16] } b2
\afterGrace b2 { f16[ a16] }
\appoggiatura a32 b4
\acciaccatura a8 b4
}
\new Lyrics
\lyricsto melody {
normal
\set includeGraceNotes = ##t
case,
gra -- ce case,
after -- grace case,
\set ignoreMelismata = ##t
app. case,
acc. case.
}
>>