Python won't load C-language DLL in Windows 11 (that loads in Windows 10)

I’m running Python 3.8.5 on Windows 10 Pro, Version 10.0.19045 Build 19045. I have an application where I have moved part of the processing to a DLL, written in C using Microsoft Visual Studio C++ 2019. I’m loading my library DLL and calling functions within it with no problems.

My colleagues have now started testing, and report the DLL is ‘unable to load’ under Windows 11. Any ideas why?

I have been able to duplicate the bug with the following minimal application:

test.py

import ctypes
import _ctypes

dll = ctypes.WinDLL("./test_dll.dll")
dll.RunTest.restype = ctypes.c_int
dll.RunTest.argtypes = [ctypes.c_int]
print(dll.RunTest(0))
_ctypes.FreeLibrary(dll._handle)

test.h:

#ifndef TEST_H
#define TEST_H

#if defined(_MSC_VER) && _MSC_VER>=1020
#pragma once
#endif

#include <stdint.h>
#include <windows.h>


#if !defined(MYDLL)
# if defined(TESTDLL_EXPORTS)
#  define MYDLL extern "C" __declspec(dllexport)
# else
#  define MYDLL extern "C" __declspec(dllimport)
# endif
#endif

#ifdef __cplusplus
extern "C" {
#endif

    MYDLL int WINAPI RunTest(int seed);

#ifdef __cplusplus
}
#endif

#endif

test.cpp:

#include "pch.h"
#include "test.h"

MYDLL int WINAPI RunTest(int seed) {
    return 42;
}

This prints 42 on Windows 10 and is ‘unable to load’ on Windows 11

Thanks in advance!