OK, whatever repo that is providing vivaldi-beta, either doesn't sign their rpms, or
you have not yet imported the key used to sign it
Evidence so far: not a bug
From: Neal Becker <ndbecker2(a)gmail.com>
Sent: Thursday, December 17, 2015 9:23 AM
Subject: Re: plasma software update trouble
It's just 1 package: vivaldi-beta
dnf doesn't show any update, must be using different metadata.