DynamicsStaff spacing itself too far from the music

[Richard Shann]

In a complex score with a DynamicsStaff placed above the music for
markups and another below for the dynamics an unusually large space
opened up below the music and above the dynamics.
I pared the complex score down to this MWE:

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\version "2.24"
marks = {
                %This line is needed.
                \repeat unfold 40 {s1 }     
                }
music = {
                 \repeat unfold 80 {c''1 }        
                  }
dyns = {
                     \repeat unfold 80 { <>  \f c''1 }      
                  }
\paper {
        %this line is needed.
        system-system-spacing.minimum-distance = 15
       }

\score {
          <<   \new Dynamics <<\marks >> 
               \new Staff <<   \music  >> 
               \new Dynamics << \dyns >>
           >>
       } 

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In this example the extra spacing disappears after the third system.
Strangely, the problem goes away if the system-system-spacing.minimum-
distance is not set, however this does not happen on the complex score,
and, as there is plenty of room on the page it is difficult to see how
this setting could be called into play.

Any insights into this behavior would be most welcome, as well as any
setting that might directly address this spacing.

Richard Shann