UserAgent: Mozilla/5.0 (Windows NT 6.1; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/63.0.3239.132 Safari/537.36

Steps to reproduce the problem:
1. Build an extension and do not upload it to the Chrome Web Store
2. Setup an environment so that this self-hosted extension is force-installed via the ExtensionInstallForcelist policy
3. Start Chrome via pytest-selenium (executing a .py script)

What is the expected behavior?
The extension is force-installed.

What went wrong?
If Chrome is started by the .py script, the extension is not installed.
If Chrome is started like a normal user would do it, the extension is force-installed.
This behavior occurs on the same machine.

Did this work before? N/A 

Chrome version: 63.0.3239.132  Channel: stable
OS Version: 
Flash Version: 

The command line from the chrome://version tab:

C:\Program Files (x86)\Google\Chrome\Application\chrome.exe" --cancel-first-run --disable-background-networking --disable-client-side-phishing-detection --disable-default-apps --disable-hang-monitor --disable-popup-blocking --disable-prompt-on-repost --disable-sync --disable-web-resources --enable-automation --enable-logging --force-fieldtrials=SiteIsolationExtensions/Control --ignore-certificate-errors --load-extension="C:\Users\Test1\AppData\Local\Temp\scoped_dir7780_3898\internal" --log-level=0 --metrics-recording-only --no-first-run --password-store=basic --profile-directory="\"Default\"" --remote-debugging-port=12568 --start-maximized --test-type=webdriver --use-mock-keychain --user-data-dir="C:\Users\Test1\AppData\Local\Temp\scoped_dir7780_516" --flag-switches-begin --flag-switches-end