configuring DHCPv6 wipes out any nameservers set while configuring IPv4

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This relates to the Ubuntu 18.04 installer, I do not know what "package" this would relate to

Based on your output, I believe you are using the new Ubuntu Server installer.

I have attempted to reproduce the issue at hand, and I have managed to complete the installation with dhcpv6 + static ipv4 connectivity with DNS server propagating correctly to the installed system. The end networking info generated was:

# This file is generated from information provided by
# the datasource. Changes to it will not persist across an instance.
# To disable cloud-init's network configuration capabilities, write a file
# /etc/cloud/cloud.cfg.d/99-disable-network-config.cfg with the following:
# network: {config: disabled}
network:
    ethernets:
        ens3:
            addresses:
            - 192.168.122.7/24
            dhcp6: true
            gateway4: 192.168.122.1
            nameservers:
                addresses:
                - 192.168.122.1
                search: []
            optional: true
    version: 2

Which is what I would expect, after filling out static network configuration (subnet, ip, gateway, nameserver).

Please provide more detailed instruction on how to reproduce the issue? Or maybe inspect the system using additional tty's? And provide more detailed steps and screenshots / photos?

The environment is Hyper-V 2012r2, screenshots as attached

"This then causes the install to fail." what do you mean by this? The screenshots you've posted look fine to me.

As the IP stack is up, just not DNS, I was able to use another TTY and FTP the contents of the attached .zip to a local FTP server

If there are other files that would be more useful, please let me know.

From the type of issue, I don't see why the "hardware" would have any effect, but this is a Hyper-V 2012r2 G2 VM

Ah so I think what's happening here is that when you select "use DHCPv6", the nameservers configured when you configured IPv4 are forgotten. If you configure DHCPv6 first and then configure IPv4 does that work?

This clearly isn't sensible behaviour.

after changing to use DHCPv6 and going back, the DNS servers and search domain are empty

after adding DNS servers and search domain and then continuing, the full log now shows the nameservers and search domain, the install completes, and I have IPv4 and IPv6 connectivity with working DNS

Going back and re-entering DNS servers and search domain during the install is a _workable_ method.

I would have thought that static V4 with DHCPv6 was not _that_ uncommon.

Thanks for checking. I wasn't meaning to propose what you did as a solution, although it just about qualifies as a workaround I guess, but by doing that you confirmed my suspicions as to what the problem was!