Why curl_easy_setopt in libcurl does not take effect immediately?

I'm writing a network library based on cpr and libcurl library.

In post function, I wrote something like this:

cpr::Response  r;
{
    std::lock_guard<std::mutex> lock(m_lockPostClient);
        
    if (m_doh == "on") {
        curl_easy_setopt(m_postClient.GetCurlHolder()->handle, CURLOPT_DOH_URL, m_doh_url.c_str());
    }
        
    m_postClient.SetTimeout(cpr::Timeout((int)timeout));
    m_postClient.SetConnectTimeout(cpr::ConnectTimeout((int)timeout));
    m_postClient.SetUrl(cpr::Url{ url });
    m_postClient.SetHeader(header);
    m_postClient.SetBody(cpr::Body(body));
    m_postClient.SetVerifySsl(cpr::VerifySsl(true));

        
    r = m_postClient.Post();
}

Which m_postClient is the cpr::Session variable. m_doh and m_doh_url sets the DoH when posting.

However, after curl_easy_setopt function, it seems like DoH does not take effect immediately in the Post() following. BTW, it does not looks like two different threads.

I need to wait sometime to make the DoH setting taken effect. It will taken effect after several post request. It really confused me, could anyone tell me reason?