Why is stderr of a custom_target captured/hidden?

[amezin]

I run gtk-builder-tool as a custom_target:

  pref_ui_gtk3_files += custom_target(
    command: [gtk3_builder_tool, 'simplify', '@INPUT@'],
    input: pref_ui_file,
    output: '@PLAINNAME@',
    capture: true,
    ...
  )

When I run gtk-builder-tool simplify filename.ui > output.ui in the terminal manually, I see some warnings. When I build with meson and ninja, however, I see nothing.

Looking at the code

meson/mesonbuild/scripts/meson_exe.py

Line 68 in f1035e0

if p.returncode != 0:

I see that the output is printed to the terminal only if the subprocess terminates with non-zero exit code.

So, why is it hiding non-fatal issues? Why touch stderr at all?