[DiapDealer]

I've rarely found a need for shell=True on Windows myself.

The described behavior still makes me think that part of PyInstaller's PyQt6 freezing strategy involves setting some Qt environment variable(s). You could try dumping the environment to a msgbox (before the subprocess call to launch Sigil) and look for something like QT_QPA_PLATFORM_PLUGIN_PATH. That would definitely cause issues if set. If it is, you could pass a sanitized env parameter to the subprocess.Popen call with all Qt/Python environment variables removed.

Something else to try is passing the cwd parameter to the subprocess.Popen call and setting it to Sigil's program directory (making sure to use a relative path to Sigil's exe rather than the full path).