Why doesn't Ubuntu 18.04 keep network config in place after install?

I am installing 18.04 Server with the network mini.iso as a VM and using IPv6 DHCP. I have no IPv4 WAN addresses available.

The installation works fine. I give the installer the DNS servers and it configures the rest over DHCP. All packages are downloaded OK over the WAN during the install (at least, I assume they are!). However, when the installation finishes, I have no network and I have to configure it manually. Note that networking is configured after install (as IPv6 DHCP in my case), but with no DNS servers.

Why is this? Should not the installer apply the network configs it used?