So this might be the thing happened.
First, my assumption is that the Riven only have +Critical Chance and +Reload Speed or no elemental on the riven.
With that out of the way, let me start explaining.
The Mutalist Cernos has 2 sources of Status proc: The arrow shot itself, and the Toxin cloud created by the shot.
For the arrow, based on the mod setup, the elemental combo are: Electricity - Heat - Cold which become Radiation - Cold.
However, on the Toxin cloud, it has Electricity - Heat - Cold - Toxin, which become Radiation - Viral. (with the guaranteed Toxin)
So that's why there's a Cold proc separated even when there's innate Viral on modding screen, since the game try to display all elemental you can proc with the bow.
This is not a bug. It's how the elemental system work (weird, right?).
Also sorry if the explanation might be hard to understand, I'm not a native English speaker nor having good score on English.