\version "2.25.3"

global = {
  \key c \major
  \time 4/4
}

right = \relative c'' {
  \global
  % Music follows here.

  c4 d e f | g f e d | c1
}

left = \relative c' {
  \global
  % Music follows here.

  c4 d e f | g f e d | c1
}

\score {
  %% This could also be a GrandStaff
  \new PianoStaff <<
    %% It must contain two staves to show the problem
    \new Staff = "right" \right
    \new Staff = "left" { \clef bass \left }
  >>
  \layout { }
}