Rengar 5 Stacks Bug

I was blue-side Rengar, I was duelling enemy Rengar in river, and i E'd on 4 stacks resulting in 5 stacks. Now, at 5 stacks my E was on CD, this isn't supposed to happen on a normal bias due to Rengar's 5 stacks clears you of all CD's on the moment (unless you use a spell and the normal spell CD is supposed to happen). This is the second time this happened to me and this time it costed me a Death