Rengar (R) Empowered [Cooldown BUG]

I don't know if anybody noticed, but this happend to me once, and some weeks ago it happened to me again, for some reason when i tryed to get my second empowered ebola with my ult for some reason it was still on couldown after my ult, even knowing it was retrieving stack by the ult at them max is this normal ? Because i realy think it is a bug