First two bytes of 'stdout' are lost

I am trying to use StringIO to capture stdout, in code that looks like this:

import sys
from io import StringIO
old_stdout = sys.stdout
sys.stdout = mystdout = StringIO()
print( "patate")
mystdout.seek(0)
sys.stdout = old_stdout
print(mystdout.read())

Well, it is not exactly like this, since this works properly

This code is actually run from C++ using the C Python API.
This worked quite well, so the code was right at some point. But now,
two things changed:
 - Now using python 3.11.7 instead of 3.7.12
 - Now using only the python limited C API

And it seems that now, mystdout.read() always misses the first two
characters that have been written to stdout.

My first ideas was something related to the BOM improperly truncated
at some point, but i am manipulating UTF-8, so the bom would be 3
bytes, not 2.

I ruled out wrong C++ code to extract the string from the python
variable, since running a python print of the content of mystdout in
the real stdout also misses the two first characters.

Hopefully someone has a clue on what would have changed in Python for
this to stop working compared to python 3.7?