VerticalAxisGroup minimum-Y-extent

[Daniel Johnson]

I've run convert-ly (2.7.18) on some files that are about a month old,
and it made the following change:

\set Staff.minimumVerticalExtent = ##f
is changed to
\override Staff.VerticalAxisGroup #'minimum-Y-extent = ##f

However, this does not force the staff-distance the way setting
minimumVerticalExtent did.  Consider the following example:

%%% BEGIN LILYPOND CODE %%%
\version "2.7.18"

text = \lyricmode { Hel -- lo world }

\score {
   \context ChoirStaff <<
       \context Staff = women {
           %\set Staff.minimumVerticalExtent = ##f
           \override Staff.VerticalAxisGroup #'minimum-Y-extent = ##f
           \set Staff.verticalExtent = #'(-6 . 2)
           \clef treble
           \context Voice = sop { c4 c c2 }
       }
       \context Lyrics = sopLyrics \lyricsto sop \text
       \context Staff = men {
           %\set Staff.minimumVerticalExtent = ##f
           \override Staff.VerticalAxisGroup #'minimum-Y-extent = ##f
           \set Staff.verticalExtent = #'(-2 . 6)
           \clef bass
           \context Voice = bas { c4 c c2 }
       }
    >>
}
%%% END LILYPOND CODE %%%

In 2.7.14 and earlier (and in 2.6.4), the lyrics would be placed exactly
centered between the two staves, regardless of collision with notes. 
However, in 2.7.17 the lyrics are pushed down by the low notes in the
upper staff.

Am I completely missing something, or is minimum-Y-extent not working as
advertised?

--Daniel