Could it be there are two or more separate loot tables by planet or mission? Everyone who has gotten Rime Rounds reports they have gotten it from Sedna.
Today I ran about 15 spy missions on Titan, Saturn and about 5 on Oceanum, Pluto. The drops I ended up with were 2 vicious frosts, 2 frigid blasts, and 2 thermite rounds. So perhaps one planet's spy missions give Vicious Frost, Frigid Blast, Thermite Rounds, Scorch with another planet's being Volcanic Edge, Scattering Inferno, Rime Rounds, Frostbite.