Kalista Runaan's Bug?

I have Runaan's Hurricane. I'm not sure if this is working as intended but... I have stacks of rend in one target, and I auto-attack and Activate Rend before it lands. Rend kills the target, resetting the cooldown. The extra bolts from runaan's attack a different target and when they land, rend is activated again by itself putting it on cooldown.