Why std::nounitbuf doesn't do anything on MSVC?

I am writing a course on modern C++ and wanting to explain how the flush operations work, so I decided to disable the automatic flush in order to write to the buffer only when std::cout is destroyed.

For that, only one standard function exists and it is the manipulator std::nounitbuf.

However, this doesn't work on at least MSVC.

What surprises me the most is that the alternative in C, the setvbuf function, actually allows you to deactivate the automatic flush.
It bothers me a little because I want to teach C++ and not C.

Can anyone help me? thank you!

#include <iostream>
#include <thread>

int main()
{
    std::cout << std::nounitbuf; // doesn't work (output is two Hello World before the 5 seconds delay)
    //setvbuf(stdout, nullptr, _IOFBF, BUFSIZ); // work (output is one Hello World, then another one after the 5 seconds)

    std::cout << "Hello World" << std::endl;
    std::cout << "Hello World";

    std::this_thread::sleep_for(std::chrono::seconds{ 5 });
}