On the mobile platforms (Android, iOS and UWP) calling QNetworkProxyFactory::setUseSystemConfiguration(true) does not read and force the system proxy settings to be used.
If you call QNetworkProxyFactory::proxyForQuery() after setting the usesSystemConfiguration to true, it continues to return QNetworkProxy::NoProxy. Calling post in a QNetworkAccessManager does not go through the proxy. This is not the case on Windows Desktop or Linux.
I have attached a simple app that displays the proxy list that is being used by the app after calling the above set method.