Ok, i can reproduce the issue, and while debugging it i found something else:

1) flash-kernel is broken

...
ubuntu@ubuntu:~$ sudo dpkg -i linux-image-4.15.0-1029-raspi2_4.15.0-1029.31~1806108take2_arm64.deb
dpkg: warning: downgrading linux-image-4.15.0-1029-raspi2 from 4.15.0-1029.31 to 4.15.0-1029.31~1806108take2
(Reading database ... 61762 files and directories currently installed.)
Preparing to unpack linux-image-4.15.0-1029-raspi2_4.15.0-1029.31~1806108take2_arm64.deb ...
Unpacking linux-image-4.15.0-1029-raspi2 (4.15.0-1029.31~1806108take2) over (4.15.0-1029.31) ...
Setting up linux-image-4.15.0-1029-raspi2 (4.15.0-1029.31~1806108take2) ...
Processing triggers for linux-image-4.15.0-1029-raspi2 (4.15.0-1029.31~1806108take2) ...
/etc/kernel/postinst.d/initramfs-tools:
update-initramfs: Generating /boot/initrd.img-4.15.0-1029-raspi2
flash-kernel: deferring update (trigger activated)
raspi3-firmware: deferring update via initramfs-tools (hook will be called later by the kernel package)
/etc/kernel/postinst.d/x-grub-legacy-ec2:
Searching for GRUB installation directory ... found: /boot/grub
Searching for default file ... Generating /boot/grub/default file and setting the default boot entry to 0
Searching for GRUB installation directory ... found: /boot/grub
Testing for an existing GRUB menu.lst file ... 

Could not find /boot/grub/menu.lst file. Would you like /boot/grub/menu.lst generated for you? (y/N) /usr/sbin/update-grub-legacy-ec2: line 1101: read: read error: 0: Bad file descriptor
run-parts: /etc/kernel/postinst.d/x-grub-legacy-ec2 exited with return code 1
dpkg: error processing package linux-image-4.15.0-1029-raspi2 (--install):
 installed linux-image-4.15.0-1029-raspi2 package post-installation script subprocess returned error exit status 1
Processing triggers for flash-kernel (3.90ubuntu3) ...
Unsupported platform.
dpkg: error processing package flash-kernel (--install):
 installed flash-kernel package post-installation script subprocess returned error exit status 1
Errors were encountered while processing:
 linux-image-4.15.0-1029-raspi2
 flash-kernel
...

2) i saw a couple of systemd services fail:

ubuntu@ubuntu:~$ systemctl status systemd-modules-load.service
● systemd-modules-load.service - Load Kernel Modules
   Loaded: loaded (/lib/systemd/system/systemd-modules-load.service; static; vendor preset: enabled)
   Active: failed (Result: exit-code) since Sun 2018-01-28 15:58:19 UTC; 10 months 5 days ago
     Docs: man:systemd-modules-load.service(8)
           man:modules-load.d(5)
  Process: 676 ExecStart=/lib/systemd/systemd-modules-load (code=exited, status=1/FAILURE)
 Main PID: 676 (code=exited, status=1/FAILURE)

Jan 28 15:58:19 ubuntu systemd-modules-load[676]: Inserted module 'iscsi_tcp'
Jan 28 15:58:19 ubuntu systemd-modules-load[676]: Failed to find module 'ib_iser'
Jan 28 15:58:19 ubuntu systemd[1]: systemd-modules-load.service: Main process exited, code=exited, status=1/FAILURE
Jan 28 15:58:19 ubuntu systemd[1]: systemd-modules-load.service: Failed with result 'exit-code'.
Jan 28 15:58:19 ubuntu systemd[1]: Failed to start Load Kernel Modules.

this one seems just noise, while i don't know about this one:

ubuntu@ubuntu:~$ systemctl status systemd-tmpfiles-setup.service
● systemd-tmpfiles-setup.service - Create Volatile Files and Directories
   Loaded: loaded (/lib/systemd/system/systemd-tmpfiles-setup.service; static; vendor preset: enabled)
   Active: failed (Result: exit-code) since Sun 2018-01-28 15:58:24 UTC; 10 months 5 days ago
     Docs: man:tmpfiles.d(5)
           man:systemd-tmpfiles(8)
  Process: 1066 ExecStart=/bin/systemd-tmpfiles --create --remove --boot --exclude-prefix=/dev (code=exited, status=1/FAILURE)
 Main PID: 1066 (code=exited, status=1/FAILURE)

Jan 28 15:58:24 ubuntu systemd[1]: Starting Create Volatile Files and Directories...
Jan 28 15:58:24 ubuntu systemd-tmpfiles[1066]: ACL operation on "/var/log/journal" failed: Invalid argument
Jan 28 15:58:24 ubuntu systemd-tmpfiles[1066]: ACL operation on "/var/log/journal" failed: Invalid argument
Jan 28 15:58:24 ubuntu systemd[1]: systemd-tmpfiles-setup.service: Main process exited, code=exited, status=1/FAILURE
Jan 28 15:58:24 ubuntu systemd[1]: systemd-tmpfiles-setup.service: Failed with result 'exit-code'.
Jan 28 15:58:24 ubuntu systemd[1]: Failed to start Create Volatile Files and Directories.