I was able to reproduce the problem with the steps. While the function was supposed to order the include dirs automatically it fails to do so correctly on non-Windows platforms where the separator for the paths in AMENT_PREFIX_PATH is : (instead of ; as on Windows and in CMake).
Please see https://github.com/ament/ament_cmake/pull/157 for the proposed patch. You should be able to apply the same diff to the file installed in /opt/ros/crystal/share/ament_cmake_include_directories/cmake/ament_include_directories_order.cmake and your next build should succeed.