[Gameplay] Kalista Rend Bug - Strange "recast"

So... I was playing in a normal game, and was peacefully farming. But then, after i killed 2 creeps with my rend, it started to bug. If i last-hit a minion with it, the cooldown is not reset, but the mana cost is and the audio queue is not played. Also, my next auto attack will use Rend directly on hit (and the spear will disappear), apply the damage and then reset the cooldown.

The bug lasted for aproximately 2 mins, until i hit an enemy champion and used my Rend to deal damage (but not reset it).

I don't know exactly how to reproduce the bug, since i've "double killed" minions quite ofter after this, and the bug didn't reproduce.

Hope the video i linked helps to see what i mean