nx_ip_link_status_change_notify_set not working

I registered a link_status_change callback, but it doesn't seem to be called. The IP interface is up, but I don't enter the callback, even when the ethernet is unplugged. I'm using the NXP iMXRT1050.
VOID link_status_change_cb (NX_IP ip_ptr, UINT interface_index, UINT link_up) {
ULONG ip_address, network_mask;
__BKPT(0);
if (NX_SUCCESS == nx_ip_interface_address_get (ip_ptr, interface_index, &ip_address, &network_mask)) {
if (NX_TRUE == link_up) {
PRINTF("IP is up!");
char ipString[17];
getIpV4String(ip_address, ipString);
PRINTF("IP is %s\r\n", ipString);
}
else if (NX_FALSE == link_up) {
PRINTF("IP is down!");
}
}
}
void initNet(void){
nx_system_initialize();
// Create a packet pool.
status = nx_packet_pool_create(&packet_pool_0, "NetX Main Packet Pool", PACKET_SIZE+64,
(ULONG ) (((int ) packet_pool_area + 15) & ~15), NX_PACKET_POOL_SIZE);
if (status) {
//DbgConsole_Printf("ERR: packet pool \r\n");
}
// Create an IP instance.
status = nx_ip_create(&ip_0, "NetX IP Instance 0", ip_addr, 0xFFFFFF00UL, &packet_pool_0,
nx_driver_imx, (UCHAR* ) ip_thread_stack, sizeof(ip_thread_stack), 5);
if (status) {
__BKPT(0);
}

status = nx_ip_link_status_change_notify_set(&ip_0, link_status_change_cb);  
if (status) {  
		__BKPT(0);  
}  

}