I believe there is a issue with the way LilyPond handles grace notes on ledger lines. As far as I know, ledger lines should be always visible and nothing must block them (beams, slurs, slashes, etc.). LilyPond does that automatically for regular notes, but the beams and slashes of grace notes do cover the ledger lines. See the code below:
I tried looking on Elaine Gould's Behind Bars for a reference on the issue. While I could not find such a statement about grace notes, the following excerpts make clear that in her view the ledger lines cannot be obstructed:
- on p. 126, she writes: "Ensure that the grace note on ledger lines has a sufficiently long stem for the diagonal stroke [slash] not to obscure a ledger line."
- on p. 7, she uses this example which shows a long stem for a grace note:
- on p. 129, she writes aboust slurred grace notes: "A slur must not obscure a ledger line."
- on p. 16, she writes: "The outer stave-line [ledger lines] must be clearly visible; tails [flags] closer to noteheads will obscure ledger lines"
- then on p. 26, she gives the following example using grace notes, clearly showing the extended stem to avoid covering the ledger lines with the beams:
This is what LilyPond outputs with the same music as above:
I believe that it's safe to say that (in her opinion) the ledger lines must stay all time clear of beams.
Finally, just for the sake of comparison, this is how Sibelius handles the issue: