I have run into the situation where this symlink still points to the wrong place, which would cause any of the RPMs to fail the check. Look to see if the symlink is actually pointing where it should. I've also seen it try to use /etc/pki/rpm-gpg/RPM-GPG-KEY-fedora-x86_64, so check to see that this one points to the correct Fedora version's key.
There are clearly some bugs in dnf system-upgrade where setting up the GPG keys isn't done correctly.