Witchbane Jewel + Hex Master Passive = Not Curse Immune?

I am using the Witchbane unique jewel that says:

"When you Kill an Enemy Cursed with a Non-Aura Hex, become Immune to
Curses for remaining Hex Duration."

I am using the Hex Master passive that says:

"Your Hexes have infinite Duration. 20% less Effect of your Curses."

I am using a 4-Link Arcanist Brand with 3 curses: Punishment, Elemental Weakness, Assassin's Mark.

I am also using Profane Prozy unset ring + Skitterbots + Frostbite curse (but this applies the curse as an aura).

When I use Arcanist Brand, I see the multiple curses on the monsters, and the curse last forever. However when I kill them, I don't gain any curse immunity. I can easily see this because my Skitterbots are also applying the Frostbite curse to me.

What is going wrong here? Why am I not curse immune?

Are you using traps, mines, totems, or minions to kill enemies?

Yes, I am using Eye of Winter turned into a mine with High Impact Mine Support.

Ah... Thank you for the explanation.