Ubuntu 18.04 Pro订阅环境下unattended-upgrades无法处理ESM安全更新的问题求助

大家好，我遇到一个棘手的问题，想请教下各位：

我有一台Ubuntu 18.04的机器，已经绑定了Ubuntu Pro订阅令牌，通过sudo pro security-status可以看到系统有不少待处理的ESM安全更新，具体输出如下：

763 packages installed:

680 packages from Ubuntu Main/Restricted repository

57 packages from Ubuntu Universe/Multiverse repository

26 packages from third parties

To get more information about the packages, run

pro security-status --help

for a list of available options.

The system apt cache may be outdated. Make sure to run

sudo apt-get update

to get the latest package information from apt.

This machine is attached to an Ubuntu Pro subscription.

Main/Restricted packages are receiving security updates from

Ubuntu Pro with 'esm-infra' enabled until 2028. There are 72 pending security updates.

Universe/Multiverse packages are receiving security updates from

Ubuntu Pro with 'esm-apps' enabled until 2028. There are 10 pending security updates.

但奇怪的是，当我运行sudo unattended-upgrade -v尝试自动处理这些更新时，结果显示找不到可升级的包，输出如下：

Initial blacklisted packages:

Initial whitelisted packages:

Starting unattended upgrades script

Allowed origins are: o=Ubuntu,a=bionic, o=Ubuntu,a=bionic-security, o=UbuntuESM,a=bionic

No packages found that can be upgraded unattended and no pending auto-removals

我们的需求是通过unattended-upgrades自动处理这些ESM安全更新，而不想直接执行全量的sudo apt upgrade。我已经检查过unattended-upgrades的配置文件，里面的Allowed-Origins配置看起来是正确的：

Unattended-Upgrade::Allowed-Origins {
    "${distro_id}:${distro_codename}";
    "${distro_id}:${distro_codename}-security";
    // Extended Security Maintenance; doesn't necessarily exist for
    // every release and this system may not have it installed, but if
    // available, the policy for updates is such that unattended-upgrades
    // should also install from here by default.
    "${distro_id}ESM:${distro_codename}";
    //      "${distro_id}:${distro_codename}-updates";
    //      "${distro_id}:${distro_codename}-proposed";
    //      "${distro_id}:${distro_codename}-backports";
};

我实在搞不清楚哪里出问题了，请问各位我是不是遗漏了什么配置或者步骤，才导致unattended-upgrades无法识别这些ESM的安全更新呢？

备注：内容来源于stack exchange，提问作者yi1