Comment 13 for bug 1799319

It does seem that check-new-release-gtk will silently exit if there are available updates. However, if you run it via the command line you'll see the error.

 $ /usr/lib/ubuntu-release-upgrader/check-new-release-gtk -d
Checking for a new Ubuntu release
Please install all available updates for your release before upgrading.

So the next step would be to see if the thing that calls check-new-release-gtk is aware of whether there are updates available.