Excessive spacing between staves with \MarkLine

[Cantus Ornatus]

Hi everyone.
In typesetting chamber music with accompaniment, I would like to print
the tempo marks on the bottom staff.

Unfortunately, a tempo change produces a lot of space upon the bottom
staff, until the last tempo mark.

I think that a similar issue has already been discussed two years ago
<http://lilypond.1069038.n5.nabble.com/Possible-bug-with-tempo-mark-spacing-td218330.html>
but I did not understand how to work it out.

Here is an example.
Thanks in advance,
Francesco

%%%%%%%%%%%%%%%%%%

\version "2.20.0"

global = {
  \tempo "Adagio"
  s1*30
  \tempo "Allegro"
  s1*2
}

music = \relative c'' {
  \repeat unfold 33 { a1 }
}


\layout {
  \context {
    \name "MarkLine"
    \type "Engraver_group"
    \consists Output_property_engraver
    \consists Axis_group_engraver
    \consists Mark_engraver
    \consists Metronome_mark_engraver
    \override RehearsalMark.extra-spacing-width = #'(0 . 1)
    \override MetronomeMark.extra-spacing-width = #'(0.5 . 0)
    \override VerticalAxisGroup.minimum-Y-extent = #'(-2 . 2)
    \override VerticalAxisGroup.staff-staff-spacing =
    #'((basic-distance . 1)
       (minimum-distance . 1)
       (padding . 1)
       (stretchability . 3))
  }
  \context {
    \Score
    \remove Mark_engraver
    \remove Metronome_mark_engraver
    \accepts MarkLine
  }
}


\score {
  <<
    \new Staff << \global \music >>
    \new Staff << \global \music >>
    \new MarkLine \global
    \new Staff << \global \music >>
  >>
}

%%%%%%%%%%%%%%%%%%