`StaffGroup` influences spacing of `DynamicText`

[Werner LEMBERG]

Can someone please explain to me why the 'p sub.' in the first system
is badly spaced compared to the second one?  How can I avoid this?
Actually, this smells like a bug...

```
\version "2.25.2"

psub = \tweak DynamicText.self-alignment-X #-0.7
       #(make-dynamic-script
          #{ \markup { \dynamic "p" \normal-text \italic "sub." } #})

\score {
  \new StaffGroup <<
    \context Staff = "x" {
      f'8 f' f' f' f' f' f' f'\psub
      f'4 f' f' f'
    }
    \context Staff = "x" { s1*2 }
  >>
}

\score {
  <<
    \context Staff = "x" {
      f'8 f' f' f' f' f' f' f'\psub
      f'4 f' f' f'
    }
    \context Staff = "x" { s1*2 }
  >>
}
```


    Werner