note-by-number markup command doesn’t scale the stem properly

Reported by Ben Rudiak-Gould:

\version "2.21.2"  
\book {  
  \paper { #(layout-set-staff-size 40) }  
  { \tempo 8=88 c'8 }  
}  

The stem in the markup is twice as large (horizontally and vertically) as it should be. While fiddling around with that example I also noticed that set-global-staff-size has the opposite effect, making the stem too small if you set a size larger than 20:

\version "2.21.2"  
\book {  
  #(set-global-staff-size 40)  
  { \tempo 8=88 c'8 }  
}  

set-global-staff-size doesn't work inside \book anyway so I suppose it doesn't matter, but I'd still expect the stem size to be consistent with everything else in this example if it was being calculated correctly.

Since everything else, including note heads and flags, staff and ledger lines, stems of ordinary non-markup notes, etc., scales consistently, I'd think there must be already working code somewhere that could be copy-pasted into note-by-number.