Comment 6 for bug 1979238

I found a workaround: create a file /etc/apt/apt.conf.d/90update-manager-use-apt-phasing with the following contents -

Update-Manager {
  Always-Include-Phased-Updates true;
}
APT {
  Get {
    Always-Include-Phased-Updates false;
  }
}

As discussed in this ubuntuforums thread: https://ubuntuforums.org/showthread.php?t=2478639&p=14116259&viewfull=1#post14116259

In case someone who wants to always install all available updates regardless of phasing is reading this: in your case, use the following instead -

Update-Manager {
  Always-Include-Phased-Updates true;
}
APT {
  Get {
    Always-Include-Phased-Updates true;
  }
}