It seems like Apple has changed something in the network infrastructure. Code compiled with Qt 5.13.0 Beta4 works correctly in 10.14.2 and reports the correct interface type for all network interfaces.
When the networkinterfaces is retrieved for the first time the type is correct and shows e.g. Wifi or Ethernet. The second time the same interfaces are of type "unkown"
Attached an small example which retrieves the networkinterfaces and prints them with their type in two for loops