I’m not familiar with the C++ code but I’m with you: the incoming request is clearly GET method. You could try writing some unit tests or using Insomnia/Postman to verify your app’s functionality apart from the C++ client.
One suggestion: try capturing the entire Request to the logs to see if there are any other “gotchas” in there. Something like from your controller:
log_message('debug', print_r($this->request, true));
