install CXX_MODULES_BMI has stage behaviour

Development
1

I struggle over the CXX_MODULES install BMI. Sometimes this has no effect or some files are missing?

I use always this cmake install function call:

            install(
                TARGETS "${_tgt}"
                EXPORT ${BOOST_EXPORT_NAME}
                ARCHIVE
                    ${_lib_install_dir}
                    COMPONENT "${install_component_name}_Development"
                LIBRARY
                    ${_lib_install_dir}
                    COMPONENT "${install_component_name}_Runtime"
                    NAMELINK_COMPONENT "${install_component_name}_Development"
                RUNTIME
                    ${_bin_install_dir}
                    COMPONENT "${install_component_name}_Runtime"
                ${_install_header_set_args}
                FILE_SET ${_module_sets}
                    DESTINATION "${BOOST_DESTINATION}"
                    COMPONENT "${install_component_name}_Development"
                # NOTE: There's currently no convention for this location! CK
                CXX_MODULES_BMI
                    DESTINATION
                        ${_config_install_dir}/bmi-${CMAKE_CXX_COMPILER_ID}_$<CONFIG>
                    COMPONENT "${install_component_name}_Development"
            )


/Users/clausklein/.local/lib/cmake/modules_triangle/
├── cxx-modules
│   ├── cxx-modules-modules_triangle-targets-Release.cmake
│   ├── cxx-modules-modules_triangle-targets.cmake
│   ├── target-library__a-Release.cmake
│   └── target-library__c-Release.cmake
├── modules
│   ├── library_a.cppm
│   └── library_c.cppm
├── modules_triangle-config-version.cmake
├── modules_triangle-config.cmake
├── modules_triangle-targets-release.cmake
└── modules_triangle-targets.cmake

3 directories, 10 files
iMac:modules-triangle clausklein$

or i.e. 2 CXX_MODULES, but only 1 BMI?

bash-5.3$ tree /Users/clausklein/.local/lib/cmake/modules_playground
/Users/clausklein/.local/lib/cmake/modules_playground
├── bmi-Clang_Release
│   └── mylib.pcm
├── cxx-modules
│   ├── cxx-modules-modules_playground-targets-Release.cmake
│   ├── cxx-modules-modules_playground-targets.cmake
│   ├── target-play__hello-Release.cmake
│   └── target-play__mylib-Release.cmake
├── modules
│   ├── M.cppm
│   ├── interface_part.cppm
│   └── mylib.cppm
├── modules_playground-config-version.cmake
├── modules_playground-config.cmake
├── modules_playground-targets-release.cmake
└── modules_playground-targets.cmake

4 directories, 12 files
bash-5.3$