Not Sure If Kalista Bug or Working as Intended, but Its Something I would Like to know!

Ok, so when you are playing Kalista and you buy Runaan's Hurricane, if you kill a target that was only affected by runaans rend stacks, it will not reset your cooldown with rend. Is this a bug or is this intended?

To be more clear, say we have minion 1 2 and 3. I attack minion 2, and minion 1 and 3 get hit by runaans and obtain a stack of my rend. I attack again, and kill minion 2, so minions 1 and 3 are low and have 2 rend stacks on them. When I press e and kill minions 1 and 3 with rend from the runaans, it does not reset the cooldown on rend.

In the E description it says "Rends cooldown will be reset when the target is killed with rend stacks." If this is the case, then technically the rend cooldown should still be reset.

So let me know if this is a bug or how it should be, because if its a bug I feel like its very detrimental to Kalista, especially late game when you rend with intentions of resetting and it goes on cooldown mid fight. Thank you!