Taric ult + Sion passive bug

I don't have a clip for this cuz I'm lazy but here it is:

I recently played against a duo bot lane of and . At one point Sion had died and then Taric ults (Taric's ult was going off of both him and passive sion). But then after about .5 seconds after Taric casted ult, Sion's passive self died. When this happened Taric's ult instantly gave the undamagable buff to him, while the animation was still above him.