ASIO with Win32: Why is ReadFile blocking after a windows::object_handle::async_wait?

Question

I'm trying to read asynchronously from stdin using ASIO on windows.

To this end, I tried to use an asio::windows::object_handle.

I would expect that after async_waiting on the STDIN handle, ::ReadFile on the same handle should not block.

However, it seems that this is not the case. In the following program, I would expect echos of my input from the first coroutine, interlaced with the test n printouts from the second.

Instead, the ::ReadFile call is blocking the io_context.run(), so the test n prints only happen after I presss return in the console.

#include <sdkddkver.h>

#include <chrono>
#include <iostream>

#include "asio/asio.hpp"

int main() {
  using namespace std::chrono_literals;

  asio::io_context ctx;
  asio::high_resolution_timer timer(ctx);
  asio::windows::object_handle stdin_handle(ctx,
                                             ::GetStdHandle(STD_INPUT_HANDLE));

  /// stdin loop
  asio::co_spawn(
      ctx,
      [&]() -> asio::awaitable<void> {
        for (;;) {
          co_await stdin_handle.async_wait(asio::use_awaitable);
          char buf[100];
          DWORD n;

          //  this is blocking?
          ReadFile(stdin_handle.native_handle(), buf, sizeof(buf), &n, NULL);

          std::cout << "input (" << n << "): " << std::string_view(buf, n);
        }
      },
      asio::detached);

  /// stdout loop
  asio::co_spawn(
      ctx,
      [&]() -> asio::awaitable<void> {
        for (int i = 0; i < 10; i++) {
          std::cout << "test " << i << "\n";
          timer.expires_after(1s);
          co_await timer.async_wait(asio::use_awaitable);
        }
      },
      asio::detached);

  ctx.run();

  return 0;
}

What am I doing wrong? Does object_handle::async_wait wait for things other than actual input? How do I accomplish this?