I'm engraving Bach Invention 4 (D Minor) BWV775 in Lilypond and the existing score has grace notes (measure 37) beamed from a regular note:
My reading of https://lilypond.org/doc/v2.21/Documentation/notation/beams#manual-beams is that grace note beams can be in parallel but are seperate.
So my best attempt at present is:
a16[ d,-1] << { b8.-2] a16-. } \\ { b8. \grace { c32 b8 }} >>
which gives me:
Trying a beam into the \grace { }
gives me unterminated beam error, so it appears grace notes are beamed seperately. I suspect there may be a different approach than using a seperate voice for the grace notes though.