The default application proxy returned by QNetworkProxy::applicationProxy();

used to be of type QNetworkProxy::NoProxy.  However, it appears that somewhere between 5.7 and 5.9.1 this has changed.

It's now returning QNetworkProxy::HttpProxy.

Given this code:

#include <QCoreApplication>
#include <QNetworkAccessManager>
#include <QNetworkProxy>
#include <QDebug>

int main(int argc, char *argv[])
{
    QCoreApplication a(argc, argv);

    QNetworkAccessManager man;
    QNetworkProxy proxy = man.proxy();
    qDebug () << proxy.type();

    QNetworkProxy appProxy = QNetworkProxy::applicationProxy();
    qDebug () << appProxy.type();

    return 0;//a.exec();
}

On 5.7 I get: 0 and 2 in the output.
On 5.9.1 I get 0 and 3 in the output.

This caused some of our connections to report "Host not found" because we had never explicitly set the proxy on the network access manager.