xenial2bionic release upgrade loses DNS: empty resolv.conf

Note: I'm not sure which package better represents this bug, so I used systemd, because /etc/resolv.conf had no "nameserver 127.0.0.53" entry after the upgrade.

I did a plain standard lxd deploy of xenial, using the default images. Upgraded it to the latest xenial packages, and then issued a release upgrade to bionic.

At the end of the upgrade process, rebooted, and when the system came back up, /etc/resolv.conf had no nameserver entries:

root@xenial2bionic:~# cat /etc/resolv.conf
# Dynamic resolv.conf(5) file for glibc resolver(3) generated by resolvconf(8)
# DO NOT EDIT THIS FILE BY HAND -- YOUR CHANGES WILL BE OVERWRITTEN
# 127.0.0.53 is the systemd-resolved stub resolver.
# run "systemd-resolve --status" to see details about the actual nameservers.

That was its whole content. Before the upgrade, in xenial, it had:
root@xenial2bionic:~# cat /etc/resolv.conf
# Dynamic resolv.conf(5) file for glibc resolver(3) generated by resolvconf(8)
# DO NOT EDIT THIS FILE BY HAND -- YOUR CHANGES WILL BE OVERWRITTEN
nameserver 10.0.100.1
search lxd

systemd-resolve in the system after the upgrade to bionic was working just fine:
root@xenial2bionic:~# systemd-resolve --status
Global
         DNS Servers: 10.0.100.1
          DNS Domain: lxd
          DNSSEC NTA: 10.in-addr.arpa
                      16.172.in-addr.arpa
                      168.192.in-addr.arpa
                      17.172.in-addr.arpa
                      18.172.in-addr.arpa
                      19.172.in-addr.arpa
                      20.172.in-addr.arpa
                      21.172.in-addr.arpa
                      22.172.in-addr.arpa
                      23.172.in-addr.arpa
                      24.172.in-addr.arpa
                      25.172.in-addr.arpa
                      26.172.in-addr.arpa
                      27.172.in-addr.arpa
                      28.172.in-addr.arpa
                      29.172.in-addr.arpa
                      30.172.in-addr.arpa
                      31.172.in-addr.arpa
                      corp
                      d.f.ip6.arpa
                      home
                      internal
                      intranet
                      lan
                      local
                      private
                      test

Link 34 (eth0)
      Current Scopes: none
       LLMNR setting: yes
MulticastDNS setting: no
      DNSSEC setting: no
    DNSSEC supported: no

ProblemType: Bug
DistroRelease: Ubuntu 18.04
Package: systemd 237-3ubuntu7
ProcVersionSignature: Ubuntu 4.15.0-13.14-generic 4.15.10
Uname: Linux 4.15.0-13-generic x86_64
ApportVersion: 2.20.9-0ubuntu4
Architecture: amd64
Date: Wed Apr 11 17:31:08 2018
MachineType: LENOVO 20HRCTO1WW
ProcEnviron:
 TERM=xterm-256color
 PATH=(custom, no user)
 LANG=en_US.UTF-8
 SHELL=/bin/bash
ProcKernelCmdLine: BOOT_IMAGE=/vmlinuz-4.15.0-13-generic.efi.signed root=ZFS=nsnx/ROOT/ubuntu ro quiet splash vt.handoff=1
SourcePackage: systemd
UpgradeStatus: Upgraded to bionic on 2018-04-11 (0 days ago)
dmi.bios.date: 11/09/2017
dmi.bios.vendor: LENOVO
dmi.bios.version: N1MET41W (1.26 )
dmi.board.asset.tag: Not Available
dmi.board.name: 20HRCTO1WW
dmi.board.vendor: LENOVO
dmi.board.version: SDK0J40709 WIN
dmi.chassis.asset.tag: No Asset Information
dmi.chassis.type: 10
dmi.chassis.vendor: LENOVO
dmi.chassis.version: None
dmi.modalias: dmi:bvnLENOVO:bvrN1MET41W(1.26):bd11/09/2017:svnLENOVO:pn20HRCTO1WW:pvrThinkPadX1Carbon5th:rvnLENOVO:rn20HRCTO1WW:rvrSDK0J40709WIN:cvnLENOVO:ct10:cvrNone:
dmi.product.family: ThinkPad X1 Carbon 5th
dmi.product.name: 20HRCTO1WW
dmi.product.version: ThinkPad X1 Carbon 5th
dmi.sys.vendor: LENOVO