Omen of Whittling bug

I bought a Omen of Whittling for 5 div from the exchange and used it with a chaos orb on my Pandemonium Grasp Prismatic Ring.

I right clicked the omen and it highlighted the prefix "Adds 1 to 3 Physical Damage to Attacks" in orange. When I used a chaos orb on the ring nothing happened. The prefix stayed exactly the same with the same stats.

Is this a bug? I was expecting the lowest prefix to be removed or changed.