Landscape output does not work in Cairo
For example:
\version "2.25.1"
#(set-default-paper-size "a4" 'landscape)
{ c' }
In the default PS backend, this yields output that has A4 portrait size but with the music rotated.
In Cairo, the output has A4 landscape size with the output straight. In other words, the orientation of the music is right relative to the orientation of the page, but both should be rotated by 90°.