Surprising rendering order

[Roman Stawski]

In the snippet below, I'm trying to set up a simple numbering of the pieces.
This used to number the pieces in ascending order in version 2.12.[don't 
recall].
But not any more...

----
\version "2.13.34"

#(define sequence-number 0)
#(define-markup-command (score-sequence layout props) ()
  (set! sequence-number (+ sequence-number 1))
  (interpret-markup layout props
    (markup #:bold #:large (number->string sequence-number))))
        
\new Staff {
  \set Staff.instrumentName = \markup\score-sequence
  a'1
}
        
\new Staff {
  \set Staff.instrumentName = \markup\score-sequence
  b'1
}
----

Does someone have a clue of what's going on here?

Thanks for any help