Verified 1.10ubuntu5.1 on Disco.

Discos' 1.10ubuntu5 already had the fix to not fall back to mark all upgradable packages, it just received follow-up commits to all kinds of dependencies and skip held packages earlier.

root@dd-lp-1824804:~# dpkg -l unattended-upgrades | cat
Desired=Unknown/Install/Remove/Purge/Hold
| Status=Not/Inst/Conf-files/Unpacked/halF-conf/Half-inst/trig-aWait/Trig-pend
|/ Err?=(none)/Reinst-required (Status,Err: uppercase=bad)
||/ Name                Version       Architecture Description
+++-===================-=============-============-===========================================
ii  unattended-upgrades 1.10ubuntu5.1 all          automatic installation of security upgrades
root@dd-lp-1824804:~# apt-mark showhold
systemd
root@dd-lp-1824804:~# unattended-upgrade --verbose --debug
Running on the development release
Initial blacklist : 
Initial whitelist: 
Starting unattended upgrades script
Allowed origins are: o=Ubuntu,a=disco, o=Ubuntu,a=disco-security, o=UbuntuESM,a=disco
...
Checking: console-setup ([<Origin component:'main' archive:'disco-updates' origin:'Ubuntu' label:'Ubuntu' site:'archive.ubuntu.com' isTrusted:True>])
adjusting candidate version: console-setup=1.178ubuntu12
Checking: console-setup-linux ([<Origin component:'main' archive:'disco-updates' origin:'Ubuntu' label:'Ubuntu' site:'archive.ubuntu.com' isTrusted:True>])
adjusting candidate version: console-setup-linux=1.178ubuntu12
Checking: keyboard-configuration ([<Origin component:'main' archive:'disco-updates' origin:'Ubuntu' label:'Ubuntu' site:'archive.ubuntu.com' isTrusted:True>])
adjusting candidate version: keyboard-configuration=1.178ubuntu12
Checking: libnss-systemd ([<Origin component:'main' archive:'disco' origin:'Ubuntu' label:'Ubuntu' site:'archive.ubuntu.com' isTrusted:True>])
package libnss-systemd upgradable but fails to be marked for upgrade (E:Unable to correct problems, you have held broken packages.)
falling back to adjusting libnss-systemd's dependencies recursively
package libnss-systemd upgradable but fails to be marked for upgrade (E:Unable to correct problems, you have held broken packages.)
sanity check failed for: set()
Checking: libpam-systemd ([<Origin component:'main' archive:'disco' origin:'Ubuntu' label:'Ubuntu' site:'archive.ubuntu.com' isTrusted:True>])
package libpam-systemd upgradable but fails to be marked for upgrade (E:Unable to correct problems, you have held broken packages.)
falling back to adjusting libpam-systemd's dependencies recursively
package libpam-systemd upgradable but fails to be marked for upgrade (E:Unable to correct problems, you have held broken packages.)
sanity check failed for: set()
Checking: libsystemd0 ([<Origin component:'main' archive:'disco' origin:'Ubuntu' label:'Ubuntu' site:'archive.ubuntu.com' isTrusted:True>])
package libsystemd0 upgradable but fails to be marked for upgrade (E:Error, pkgProblemResolver::Resolve generated breaks, this may be caused by held packages.)
falling back to adjusting libsystemd0's dependencies recursively
package libsystemd0 upgradable but fails to be marked for upgrade (E:Error, pkgProblemResolver::Resolve generated breaks, this may be caused by held packages.)
sanity check failed for: set()
Checking: python3-distupgrade ([<Origin component:'main' archive:'disco-updates' origin:'Ubuntu' label:'Ubuntu' site:'archive.ubuntu.com' isTrusted:True>])
adjusting candidate version: python3-distupgrade=1:19.04.16
Checking: systemd ([<Origin component:'main' archive:'disco' origin:'Ubuntu' label:'Ubuntu' site:'archive.ubuntu.com' isTrusted:True>])
pkg systemd is on hold
sanity check failed for: set()
Checking: systemd-sysv ([<Origin component:'main' archive:'disco' origin:'Ubuntu' label:'Ubuntu' site:'archive.ubuntu.com' isTrusted:True>])
package systemd-sysv upgradable but fails to be marked for upgrade (E:Unable to correct problems, you have held broken packages.)
falling back to adjusting systemd-sysv's dependencies recursively
package systemd-sysv upgradable but fails to be marked for upgrade (E:Unable to correct problems, you have held broken packages.)
sanity check failed for: set()
Checking: ubuntu-release-upgrader-core ([<Origin component:'main' archive:'disco-updates' origin:'Ubuntu' label:'Ubuntu' site:'archive.ubuntu.com' isTrusted:True>])
adjusting candidate version: ubuntu-release-upgrader-core=1:19.04.16
pkgs that look like they should be upgraded: 
Fetched 0 B in 0s (0 B/s)                                                                                         
fetch.run() result: 0
blacklist: []
whitelist: []
Packages that will be upgraded: 
InstCount=0 DelCount=0 BrokenCount=0
Extracting content from /var/log/unattended-upgrades/unattended-upgrades-dpkg.log since 2019-05-03 17:28:55
root@dd-lp-1824804:~#