Comment 4 for bug 1802004

I'm seeing the same issue. I can reproduce it with a fresh install using http://cdimage.ubuntu.com/releases/18.04.1/release/ubuntu-18.04.1-server-amd64.iso in a virtualbox VM with all updates applied. I've tested with both the virtio, and Intel PRO/1000 network drivers with the same results.

# ls -l /etc/netplan
-rw-r--r-- 1 root root 360 Nov 9 17:12 01-netcfg.yaml
# cat /etc/netplan/01-netcfg.yaml
# This file describes the network interfaces available on your system
# For more information, see netplan(5).
network:
  version: 2
  renderer: networkd
  ethernets:
    lan0:
      match:
        macaddress: 08:00:27:3d:1a:a4
      set-name: lan0
      addresses: [10.10.10.99/24]
      gateway4: 10.10.10.1
      nameservers:
        addresses: [10.10.10.1]
# ip a
1: lo: <LOOPBACK,UP,LOWER_UP> mtu 65536 qdisc noqueue state UNKNOWN group default qlen 1000
    link/loopback 00:00:00:00:00:00 brd 00:00:00:00:00:00
    inet 127.0.0.1/8 scope host lo
       valid_lft forever preferred_lft forever
    inet6 ::1/128 scope host
       valid_lft forever preferred_lft forever
2: lan0: <BROADCAST,MULTICAST> mtu 1500 qdisc noop state DOWN group default qlen 1000
    link/ether 08:00:27:3d:1a:a4 brd ff:ff:ff:ff:ff:ff
# networkctl
IDX LINK TYPE OPERATIONAL SETUP
  1 lo loopback carrier unmanaged
  2 lan0 ether off unmanaged

2 links listed.
# netplan apply
# ip a
1: lo: <LOOPBACK,UP,LOWER_UP> mtu 65536 qdisc noqueue state UNKNOWN group default qlen 1000
    link/loopback 00:00:00:00:00:00 brd 00:00:00:00:00:00
    inet 127.0.0.1/8 scope host lo
       valid_lft forever preferred_lft forever
    inet6 ::1/128 scope host
       valid_lft forever preferred_lft forever
3: lan0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc fq_codel state UP group default qlen 1000
    link/ether 08:00:27:3d:1a:a4 brd ff:ff:ff:ff:ff:ff
    inet 10.10.10.99/24 brd 10.10.10.255 scope global lan0
       valid_lft forever preferred_lft forever
    inet6 fe80::a00:27ff:fe3d:1aa4/64 scope link
       valid_lft forever preferred_lft forever
# networkctl
IDX LINK TYPE OPERATIONAL SETUP
  1 lo loopback carrier unmanaged
  3 lan0 ether routable configured

2 links listed.

I've noticed another oddity - it seems to occasionally work correctly after a soft reboot of the VM. It seems to always exhibits the above behaviour after a cold boot of the VM.