sphinx.cmd.build.main ignores overriding of configuration setting with -D param

I'm using Sphinx command line module sphinx.cmd.build to generate documentation in multiple languages. The language setting is not stored in the conf.py, but is overridden with -D parameter in the invocation. Surprisingly the overriding works only the first time sphinx.cmd.build.main is called. All next call will ignore the setting, using the previous setting.

This worked out with old Sphinx 1.5.5 (and with old cmdline interface), but after update to newest version is no more functional.

To Reproduce
This is simple code I am using to generate the documentation (with Windows paths):

import sphinx.cmd.build
import sys


if __name__ == '__main__':
    invocation_list = [
        '-bhtml',  # builder is html
       '-Dlanguage=de',  # the additional setting,
        '..\\SphinxTest\\testproject',  # Source directory
         '..\\SphinxTest\\testproject_deutsch',  # Destination directory
    ]

    sphinx.cmd.build.main(invocation_list)

    invocation_list = [
        '-bhtml',  # builder is html
       '-Dlanguage=en',  # the additional setting
        '..\\SphinxTest\\testproject',  # Source directory
        '..\\SphinxTest\\testproject_english',  # Destination directory
    ]

    sphinx.cmd.build.main(invocation_list)
    sys.exit(0)

The document for which documentation should be generated (index.rst):

Is the frame in the correct language???
=======================================


.. toctree::
   :maxdepth: 2
   :caption: Contents:

The conf.py file

project = 'Test of Sphinx'
copyright = '2020, Company'
author = 'Company'
release = '4.0.0.0'

extensions = []
exclude_patterns = []
html_theme = 'alabaster'

Execute the script: the both HTMLs are in German (since the call was the first).

Start with the provided script and sphinx.cmd.build.main, then compare the first and second invocation using the two invocation_list values. Review how conf.py and the -Dlanguage setting are handled between calls; done means separate German and English HTML outputs are produced as requested.