undesired figured bass stretching

[Nicolas Sceaux]

Hi,

When forcing the staves spacing using
  \overrideProperty #"Score.NonMusicalPaperColumn"
  #'line-break-system-details = #'((alignment-distances . (...)))
figured bass digits are stretched, whereas they shall be tightly stacked.

I can't find how to avoid this strange behavior.  Is there a way?

Here is a minimal example:

\score {
 <<
   \new Staff {
     \overrideProperty #"Score.NonMusicalPaperColumn"
     #'line-break-system-details #'((alignment-distances . (15)))
     c'4
   }
   \new Staff <<
     { d'4 }
     \figures { <6 4>4 } >> >>
}

Nicolas