QNetworkSession does not return an interface() for mobile broadband connection on Ubuntu Desktop Linux

When I call QNetworkSession::interface() for an ethernet connection I get back a valid interface and the underlying IP address. When I call for an active mobile broadband connection I get an invalid interface. I traced through the QNetworkSessionPrivateImpl code and saw that the statement

interface = engine->getInterfaceFromId(activeConfig.identifier())

was returning "ttyUSB4", which is a device identifier. This is passed to QNetworkInterface::interfaceFromName, but the interfaces in that class are identified by interface name (e.g. "ppp0") not device id.

I tried this with the QNetworkSession isOpen() == true and isOpen() == false with no difference in behavior.