\version "2.22.2"

\header {
  dedication = \markup { \smallCaps "Title of Opera" }
}

\score {
  \new Staff {
    c1^"Act I, scene i starts here" |
    \repeat unfold 50 { c1 } | \pageBreak
    c1^"Header on this page should read “Title of Opera: Act I, scene i”" |
    \repeat unfold 50 { c } | \pageBreak
    c \bar "||"
    c^"Act I, scene ii starts here, header on this page should read “Title of Opera: Act I, scene ii”" |
    \repeat unfold 50 { c } |
  }
}