Deep Nether Creatures Traits

I am at Nether Realm 163, and all creatures I encounter have two traits. I always assumed it was an innate trait and a trait from an artifact (like my own party has) but the Decay spell did not get rid of the 2nd trait. Gem of Decay: Target’s artifact is ignored for the rest of the battle. Do they just have two innate traits at this level, or does Decay have a bug?

Enemies in Nether Realms gain an additional trait. They can’t gain traits on their artifacts though.