Culling Strike with Thrill of the kill not working as intended?

Culling Strike
Supports Attack Skills, causing them to Cull Rare and Unique Enemies on Hit

Thrill of the kill
Supports Skills you use yourself which Hit enemies. Culling a Shocked enemy with Supported Skills infuses all of your Attacks with Lightning damage for a short time.

Normally you can't visually tell if culling strike is activating but using it with Thrill of the kill you can see when it activates, because it actives thrill of the kill. According to the descriptions none of them have cooldowns, or how many times they can be activated. I'm finding that culling strike is not activating when the enemy is at X% of health. If I'm reading it correctly it should activate every time an enemy reaches X% of health. I was testing and it is so inconsistent that I'm thinking its not working as intended, or the description needs re-wording