ROS package that contains python package with a subpackage is not installed properly

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I have ROS package that contains a python package with a subpackage (nested python package). When I build the ROS package and source devel workspace, I can import the subpackage as expected. However if I source install workspace I cannot import subpackge anymore, only module of the outer python package (a.py).

Here is example file structure:

my_ws/src/ros_pkg/
    CMakeLists.txt
    package.xml
    setup.py
    src/
        my_pkg/
            __init__.py
            a.py
            my_subpkg/
                __init__.py
                b.py

Content of the setup.py file:

#!/usr/bin/env python
from distutils.core import setup
from catkin_pkg.python_setup import generate_distutils_setup

setup_args = generate_distutils_setup(
    packages=[
        'my_pkg',
    ],
    package_dir={'': 'src'}
)
setup(**setup_args)

Steps to reproduce:

Setup basic ROS package containing python package as provided in example file structure Then build it.

catkin config --install
catkin build my_pkg

Source devel workspace and try to import nested module in python (it works).

source devel/setup.bash
python -c 'from my_pkg import a'  # Works

Now source install workplace (in fresh shell) and try to import it again. It fails.

source devel/setup.bash
python -c 'from my_pkg import a'  # Works
python -c 'from my_pkg.my_subpkg import b'  # Fails (ImportError: No module named my_subpkg)

As suspected, there is no python subpackage in the install workspace (my_ws/install/lib/python2.7/dist-packages /my_pkg/)

my_ws/install/lib/python2.7/dist-packages/my_pkg/
    __init__.py
    a.py
    # my_subpkg/ is not here

I tested it on python 2 and 3 (ROS melodic and noetic).

Can anyone tell me how to achieve that the subpackage would be accessible when the install workspace is sourced?

Full story: I normally source develworkspace when working with ROS, however, I also use Industrial CI (https://github.com/ros-industrial/ind...) where tests are run based on install workspace (if I am not mistaken), which consequentially fails in that case. So if catkin cannot be configured to avoid this issue, maybe Industrial CI can be tweaked to pass tests?