It went further but still failed to update - here it is via the dbus shutdown command you supplied:

### unattended-upgrades.log:
2019-09-13 14:09:43,981 INFO Initial blacklisted packages:
2019-09-13 14:09:43,982 INFO Initial whitelisted packages:
2019-09-13 14:09:43,982 INFO Starting unattended upgrades script
2019-09-13 14:09:43,983 INFO Allowed origins are: o=Ubuntu-aptly,a=xenial
2019-09-13 14:10:01,496 INFO Packages that will be upgraded: apport apt-transport-https base-files bash bind9-host bsdutils btrfs-tools busybox-initramfs busybox-static bzip2 ca-certificates cloud-guest-utils cloud-initramfs-copymods cloud-initramfs-dyn-netconf console-setup console-setup-linux curl dbus dh-python distro-info-data dns-root-data dnsmasq-base dnsutils dpkg file friendly-recovery gettext-base git git-man gnupg gpgv grub-common grub-legacy-ec2 grub-pc grub-pc-bin grub2-common hdparm ifupdown initramfs-tools initramfs-tools-bin initramfs-tools-core iproute2 isc-dhcp-client isc-dhcp-common keyboard-configuration kmod krb5-locales libapparmor-perl libapparmor1 libasprintf0v5 libbind9-140 libblkid1 libbz2-1.0 libc-bin libcurl3-gnutls libdb5.3 libdbus-1-3 libdns-export162 libdns162 libelf1 libexpat1 libfdisk1 libgcrypt20 libglib2.0-0 libglib2.0-data libgnutls-openssl27 libgnutls30 libicu55 libisc-export160 libisc160 libisccc140 libisccfg140 libk5crypto3 libkmod2 libldap-2.4-2 liblwres141 libmagic1 libmount1 libmspack0 libpam-modules libpam-modules-bin libpam-runtime libpam-systemd libpam0g libpci3 libperl5.22 libplymouth4 libpng12-0 libpolkit-agent-1-0 libpolkit-backend-1-0 libpolkit-gobject-1-0 libprocps4 libpython3.5 libpython3.5-minimal libpython3.5-stdlib libsasl2-2 libsasl2-modules libsasl2-modules-db libseccomp2 libslang2 libsmartcols1 libsqlite3-0 libssl1.0.0 libsystemd0 libudev1 libuuid1 libx11-6 libx11-data libxml2 libxslt1.1 linux-firmware linux-headers-generic locales login lshw mount multiarch-support ntfs-3g open-iscsi open-vm-tools openssh-client openssh-server openssh-sftp-server openssl overlayroot passwd patch pciutils perl perl-base perl-modules-5.22 plymouth plymouth-theme-ubuntu-text policykit-1 procps python-apt-common python-lxml python3-apport python3-apt python3-distupgrade python3-problem-report python3-requests python3-software-properties python3-update-manager python3-urllib3 python3.5 python3.5-minimal resolvconf rsyslog shared-mime-info software-properties-common sosreport squashfs-tools sudo systemd systemd-sysv tzdata ubuntu-minimal ubuntu-release-upgrader-core ubuntu-server ubuntu-standard udev uidmap update-manager-core update-notifier-common ureadahead util-linux uuid-runtime vim vim-common vim-runtime vim-tiny vlan wget wireless-regdb xdg-user-dirs
2019-09-13 14:10:01,497 INFO Writing dpkg log to /var/log/unattended-upgrades/unattended-upgrades-dpkg.log
2019-09-13 14:10:13,935 WARNING SIGTERM received, will stop
2019-09-13 14:10:16,810 WARNING SIGTERM received, will stop
2019-09-13 14:10:19,803 WARNING SIGTERM received, will stop
2019-09-13 14:10:22,772 WARNING SIGTERM received, will stop
2019-09-13 14:10:22,779 WARNING SIGTERM received, will stop
2019-09-13 14:10:25,764 WARNING SIGTERM received, will stop
2019-09-13 14:10:25,820 WARNING SIGTERM received, will stop
2019-09-13 14:10:28,713 WARNING SIGTERM received, will stop
2019-09-13 14:10:28,873 WARNING SIGTERM received, will stop
2019-09-13 14:10:31,832 WARNING SIGTERM received, will stop
2019-09-13 14:10:34,086 WARNING SIGNAL received, stopping
2019-09-13 14:10:34,697 WARNING SIGTERM received, will stop
2019-09-13 14:10:34,779 WARNING SIGTERM received, will stop

### unattended-upgrades-shutdown.log
2019-09-13 14:09:17,147 DEBUG - Waiting for signal to start operation
2019-09-13 14:09:43,639 DEBUG - Starting countdown of 25.0 minutes
2019-09-13 14:09:43,639 DEBUG - Initializing apt_pkg configuration
2019-09-13 14:09:43,640 DEBUG - starting unattended-upgrades in shutdown mode
2019-09-13 14:09:43,645 WARNING - Running unattended-upgrades in shutdown mode
2019-09-13 14:09:43,645 DEBUG - Running plymouth --text
2019-09-13 14:09:43,657 WARNING - Unattended-upgrade in progress during shutdown, please don't turn off the computer
2019-09-13 14:09:43,657 DEBUG - Running plymouth --text
2019-09-13 14:09:46,643 WARNING - Unattended-upgrade in progress during shutdown, please don't turn off the computer
2019-09-13 14:09:46,644 DEBUG - Running plymouth --text
2019-09-13 14:09:49,647 WARNING - Unattended-upgrade in progress during shutdown, please don't turn off the computer
2019-09-13 14:09:49,648 DEBUG - Running plymouth --text
2019-09-13 14:09:52,650 WARNING - Unattended-upgrade in progress during shutdown, please don't turn off the computer
2019-09-13 14:09:52,651 DEBUG - Running plymouth --text
2019-09-13 14:09:55,654 WARNING - Unattended-upgrade in progress during shutdown, please don't turn off the computer
2019-09-13 14:09:55,654 DEBUG - Running plymouth --text
2019-09-13 14:09:58,656 WARNING - Unattended-upgrade in progress during shutdown, please don't turn off the computer
2019-09-13 14:09:58,657 DEBUG - Running plymouth --text
2019-09-13 14:10:01,660 WARNING - Unattended-upgrade in progress during shutdown, please don't turn off the computer
2019-09-13 14:10:01,660 DEBUG - Running plymouth --text
2019-09-13 14:10:04,663 WARNING - Unattended-upgrade in progress during shutdown, please don't turn off the computer
2019-09-13 14:10:04,663 DEBUG - Running plymouth --text
2019-09-13 14:10:07,667 WARNING - Unattended-upgrade in progress during shutdown, please don't turn off the computer
2019-09-13 14:10:07,667 DEBUG - Running plymouth --text
2019-09-13 14:10:10,670 WARNING - Unattended-upgrade in progress during shutdown, please don't turn off the computer
2019-09-13 14:10:10,670 DEBUG - Running plymouth --text
2019-09-13 14:10:13,674 WARNING - Unattended-upgrade in progress during shutdown, please don't turn off the computer
2019-09-13 14:10:13,674 DEBUG - Running plymouth --text
2019-09-13 14:10:13,774 WARNING - SIGTERM or SIGHUP received, stopping unattended-upgradesonly if it is running
2019-09-13 14:10:13,775 DEBUG - get_lock returned -1
2019-09-13 14:10:13,806 DEBUG - found running unattended-upgrades pid 1563
2019-09-13 14:10:13,807 WARNING - Unattended-upgrade in progress during shutdown, please don't turn off the computer
2019-09-13 14:10:13,807 DEBUG - Running plymouth --text
2019-09-13 14:10:16,675 DEBUG - get_lock returned -1
2019-09-13 14:10:16,676 DEBUG - found running unattended-upgrades pid 1563
2019-09-13 14:10:16,676 WARNING - Unattended-upgrade in progress during shutdown, please don't turn off the computer
2019-09-13 14:10:16,676 DEBUG - Running plymouth --text
2019-09-13 14:10:16,775 DEBUG - get_lock returned -1
2019-09-13 14:10:16,776 DEBUG - found running unattended-upgrades pid 1563
... more of the same ...
2019-09-13 14:10:34,697 WARNING - Unattended-upgrade in progress during shutdown, please don't turn off the computer
2019-09-13 14:10:34,698 DEBUG - Running plymouth --text
2019-09-13 14:10:34,779 DEBUG - get_lock returned -1
2019-09-13 14:10:34,779 DEBUG - found running unattended-upgrades pid 1563
2019-09-13 14:10:34,780 WARNING - Unattended-upgrade in progress during shutdown, please don't turn off the computer
2019-09-13 14:10:34,780 DEBUG - Running plymouth --text
2019-09-13 14:10:37,700 DEBUG - get_lock returned 7
2019-09-13 14:10:37,701 DEBUG - lock not taken
2019-09-13 14:10:37,701 INFO - All upgrades installed
2019-09-13 14:10:37,701 DEBUG - Running plymouth --text