Decaying Hex Mechanics Doubt

Decaying Hex only applies the Decay debuff (DoT) on ground "rune" activation of the supported curse and no other condition.

I was just wondering if that is intended or a bug, because from the skill text I got the impression it will apply the debuff when an enemy gets the curse applied on them but not estated anywhere that only the ground "explosion" can actually apply it.

Example: Using Hex Bloom will spread the curse but will not apply the debuff.