Yeah, this definitely relates to how Windows handles unqualified argv[0] in CreateProcess. Though I thought we checked that out in another issue and decided that "most" people are correctly using sys.executable here? (Or decided that it was documented well enough that they should, and using a "python" literal was relying on OS behaviour.)

I'm not a fan of trying to override the OS handling in subprocess, though that would be the fix. Possibly we could handle "python[.exe]" literals by substituting sys.executable transparently?