QNetworkAccessManager does not use requested configuration

I'm trying to determine which network interfaces/configurations have internet access.  The only way I know to do this reliably is to send a network request via an interface/configuration and receive a packet.  So here's what I'm doing:

1. Find a configuration to test via QNetworkConfiguration Manager.
2. Send a request via QNetworkAccessManger after setting it to use this configuration.
3. Handle the response.

The problem is, it's not actually using the network interface represented by the configuration.  It uses whatever interface actually has internet access, but it reports the configuration that I set.  Either I am misunderstanding how to do this, or it is failing badly.  Please see my attached test file for more info.