I can reproduce this on the live system. The package "ubuntu-extras-keyring" is installed, but "sudo apt-key list" does not show the extras signing key. A "sudo apt-get install --reinstall ubuntu-extras-keyring" fixes the issue.
I can reproduce this on the live system. The package "ubuntu- extras- keyring" is installed, but "sudo apt-key list" does not show the extras signing key. A "sudo apt-get install --reinstall ubuntu- extras- keyring" fixes the issue.