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Confusion between Xenial and Sylvia in Unattended-upgrades

I am trying to get Unattended-upgrades to work on my Linux Mint 18.3 (Sylvia) system, without success. My etc/apt/apt.conf.d/50unattended-upgrades is standard:

Unattended-Upgrade::Allowed-Origins {

but when I run it it never finds any upgrades. Here’s the output of sudo unattended-upgrades -d

Initial blacklisted packages: 
Initial whitelisted packages: 
Starting unattended upgrades script
Allowed origins are: ['o=LinuxMint,a=sylvia', 'o=LinuxMint,a=sylvia-security']
Checking: unrar ([<Origin component:'multiverse' archive:'xenial-updates' origin:'Ubuntu' label:'Ubuntu' site:'archive.ubuntu.com' isTrusted:True>, <Origin component:'multiverse' archive:'xenial-security' origin:'Ubuntu' label:'Ubuntu' site:'security.ubuntu.com' isTrusted:True>])
pkgs that look like they should be upgraded: 
Fetched 0 B in 0s (0 B/s)                                                      
fetch.run() result: 0
blacklist: []
whitelist: []
No packages found that can be upgraded unattended and no pending auto-removals

But, I have an upgrade listed in the GUI Update Manager, for unrar-free. The first line of the changelog is unrar-nonfree (1:5.3.2-1+deb9u1build0.16.04.1) xenial-security; urgency=medium

How can I make Unattended-upgrades look for xenial upgrades, as well as sylvia? Can anyone suggest a reference for the application or the configuration file? The man page is very brief.