alignment of artificial harmonics

[Graham Percival]

I'm not certain if this is a bug or not, but trying to write aritifical
harmonics (for the cello, not guitar) results in the top note of the
pair being too far to the left.

\score{\notes{ \context Voice { \clef bass \key c \minor
<ees,1 \\ {\property Voice.NoteHead \override #'style = #'diamond aes,1} >
<ees,1 \\ {\property Voice.NoteHead \override #'style = #'mensural aes,1} >
}}}

Is this a bug (where "bug" means "something not advertised", rather than
"not the best possible formatting")?  If not, what's the best way to get
the top note further to the right?  I could try spacing it out with s16
or s32, or I could use #'extra-offest, but both of those solutions seem
rather messy.

Cheers,
- Graham