Yeah, very strange that. I can only assume that it's launching the venv redirector directly, rather than the real sys.executable, and we aren't ever calling set_executable() with the real one anymore.

Dropping this into Lib/multiprocessing/spawn.py should cause a repro:

if WINSERVICE:
    _python_exe = os.path.join(sys.exec_prefix, 'python.exe')
else:
    _python_exe = getattr(sys, '_base_executable', sys.executable)

And as you point out, fixing the CreateProcess call should provide a fix.

Could you try that? And maybe submit a PR with the fix?