Python subprocess not honoring append mode for stdout on Windows

Question

On Windows, if you pass an existing file object in append mode to a subprocess, the subprocess does not really append to the file:

1. A file object with Hello World content is passed to the subprocess
2. The content is erased
3. The subprocess writes to the file
4. The expected output does not contain Hello World

Demo:

import subprocess, time, pathlib, sys
print(f'Caller {sys.platform=} {sys.version=}')

pathlib.Path('sub.py').write_text("""import sys, time
time.sleep(1)
print(f'Callee {sys.stdout.buffer.mode=}')""")

file = pathlib.Path('dummy.txt')
file.write_text('Hello World')
popen = subprocess.Popen([sys.executable, 'sub.py'], stdout=file.open(mode='a'))
file.write_text('')
time.sleep(2)
print(file.read_text())

I get the expected output

Caller sys.platform='linux' sys.version='3.8.6'
Callee sys.stdout.buffer.mode='wb'

and the unexpected bad output

Caller sys.platform='win32' sys.version='3.8.6'
NULNULNULNULNULNULNULNULNULNULNULCallee sys.stdout.buffer.mode='wb'

Is this a Python bug, and how can I make this work on Windows?

Note that the expected output is given on Windows if the file is opened in the subprocess via sys.stdout = open('dummy.txt', 'a'). So it is definitely a subprocess thing.

There is a question and another question on SO hinting at the problem without a solution for me.