`StaffGroup` prevents articulation shifted under time signature

[Werner LEMBERG]

[LilyPond version 2.23.9]

The following example

```
<<
  \new Staff { f'1 }
  \new Staff { f'1 \tweak X-offset #-5 \pp }
>>

\new StaffGroup <<
  \new Staff { f'1 }
  \new Staff { f'1 \tweak X-offset #-5 \pp }
>>
```

shows that I can't move the 'pp' to sit under the time signature if I
use `StaffGroup`.  How can I change this?  Within full scores it is of
great importance to retain this ability.


    Werner