Issue with C++ modules and gcc

Hello, the following behaves as expected with clang but not with gcc trunk. I had a look at what ninja is doing but I am not able to figure out if this a ninja or cmake or gcc issue.

CMakeLists.txt:

cmake_minimum_required(VERSION 3.28) # With 3.28, no need for set(CMAKE_CXX_SCAN_FOR_MODULES ON).
project(std_module_example CXX)

set(CMAKE_CXX_STANDARD 20)

# Default to C++ extensions being off. Clang's modules support have trouble
# with extensions right now and it is not required for any other compiler
set(CMAKE_CXX_EXTENSIONS OFF)

add_library(foo)
target_sources(foo
  PRIVATE
    foo_impl.cxx
  PUBLIC
    FILE_SET cxx_modules TYPE CXX_MODULES FILES
      foo.cxx
)
add_executable(hello main.cxx)
target_link_libraries(hello PRIVATE foo)

main.cxx:

import foo;

int main() {
    foo f;
    f.helloworld();
    return 0;
}

foo.cxx:

module;

#include <iostream>

export module foo;

export class foo {
public:
  foo();
  ~foo();
  void helloworld();
};

export void fun() {
  std::cout << "abc" << std::endl;
}

foo_impl.cxx:

module; // Start of the globl module fragment where #includes can happen

#include <iostream>

module foo;

foo::foo() = default;
foo::~foo() = default;
void foo::helloworld() { std::cout << "hello worldad  \n"; }

The issue is that the second ninja invocation below rebuilds main.o and foo.o with gcc (clang handles this correctly):

rm -rf CMakeFiles/ CMakeCache.txt build.ninja libfoo.a hello cmake_install.cmake
cmake -DCMAKE_BUILD_TYPE=Debug -GNinja
ninja
touch foo_impl.cxx
ninja -v -d explain

Versions:

cmake --version
cmake version 3.28.0

ninja --version
1.12.0.git

# Build from source...
g++ -v
gcc version 14.0.0 20231211 (experimental) (GCC)

clang -v
clang version 16.0.0

I attached the ninja outputs for clang and gcc.