Netplan doesn't configure VLAN on hotplug of underlying link

Question

We have the following netplan configuration:

network:
  version: 2
  renderer: networkd
  ethernets:
    ens3:
      dhcp4: yes
    ens9: {}
  vlans:
    ens9.4000:
      id: 4000
      link: ens9
      addresses: [ "a.a.a.a/29", "aaaa:aaaa:aaaa:aaaa::aaaa/64" ]
      gateway4: "a.a.a.1"
      gateway6: "aaaa:aaaa:aaaa:aaaa::1"

If the maschine boots up with both network devices, everything works fine:

• ens9 comes up without any address
• ens9.4000 also comes up and gets configured

If I detach the device named ens9 everything works as expected and both interfaces go down and vanish.

If I attach the device again, only ens9 comes up. ens9.4000 isn't created anymore. The same happens if the maschine comes up with one network device and the second on is hotplugged afterwards.

ens9.4000 is only created if netplan apply is called.

Are there any additional settings we are missing?

All this happens on an up to date Ubuntu 18.04.2LTS installation.

Update:

For the solution meanwhile in production I replaced netplan.io / systemd-networkd with ipupdown. Adding one somewhat special line did the trick:

allow-hotplug ens9 ens9.4000