empowered rengar e on cooldown, when reaching 5 furor stacks

I was playing Rengar in a Botgame today. One time I got my 5th furor-stack by using my E (bola strike) an tried to use the empowered e on my enemy, but it was on cooldown. And the E I used to get my 5th stack hat already rooted the enemy (as the empowered one should have done), but I could still use another empowered attack. (my Q e.g.)

I could not reproduce the bug

Hope I could help.