There are two possibly related problems. Here is a result of qDebug() << QNetworkInterface::allInterfaces(); (hardware addresses replaced with zeroes):

(QNetworkInterface(name = "lo", hardware address = "00:00:00:00:00:00", flags = IsUp IsRunning IsLoopBack , entries = ((address = QHostAddress("127.0.0.1"), netmask = QHostAddress("255.0.0.0"), broadcast = QHostAddress("127.255.255.255")), (address = QHostAddress("::1"), netmask = QHostAddress("ffff:ffff:ffff:ffff:ffff:ffff:ffff:ffff"))))
, QNetworkInterface(name = "sit0", hardware address = "00:00:00:00", flags = , entries = ())
, QNetworkInterface(name = "eth0", hardware address = "00:00:00:00:00:00", flags = IsUp CanBroadcast CanMulticast , entries = ())
, QNetworkInterface(name = "wlan0", hardware address = "00:00:00:00:00:00", flags = IsUp IsRunning CanBroadcast CanMulticast , entries = ((address = QHostAddress("192.168.1.7"), netmask = QHostAddress("255.255.255.0"), broadcast = QHostAddress("192.168.1.255")), (address = QHostAddress("fe80::ae7b:a1ff:fe74:b030%wlan0"), netmask = QHostAddress("ffff:ffff:ffff:ffff::"))))
, QNetworkInterface(name = "tun0", hardware address = "", flags = IsUp IsRunning IsPointToPoint CanMulticast , entries = ((address = QHostAddress("10.9.0.54"), netmask = QHostAddress("255.255.255.255"), broadcast = QHostAddress("10.9.0.54")), (address = QHostAddress("fe80::5192:9690:fc77:41b5%tun0"), netmask = QHostAddress("ffff:ffff:ffff:ffff::"))))
, QNetworkInterface(name = "tun0", hardware address = "", flags = IsUp IsRunning IsPointToPoint CanMulticast , entries = ())
, QNetworkInterface(name = "tun0", hardware address = "", flags = IsUp IsRunning IsPointToPoint CanMulticast , entries = ())

The list contains two duplicates of the tun0 interface without configurations. NetworkManager devices list does not contain these "ghosts" (inspected via DBus).

The second problem is that QNetworkConfigurationManager signals configurationChanged() for interface tun0 approx. 20 times per second.