mergeDifferentlyHeaded inconsistency

[address@hidden]

From Damien Pallant:

\new Staff {
\key ees \major
\clef treble
\override Staff.NoteCollision #'merge-differently-headed = ##t
\override Staff.NoteCollision #'merge-differently-dotted = ##t

\relative c' {

<< { r8 g ( bes d-3 ) f4-5 -- ees-4 -- } \\ { s8 g,~ <g bes>2. } >>
<< { r8 aes ( bes ees-4 ) f4-5 -- ees-4 -- } \\ { s8 aes,~ <aes bes>2. } >>
}
}

merge-differently-headed works on the first chord but not the second.

Is this a known issue?

Cheers,
MS