[Gameplay] Rengar E do not reset cd, when reaching 5 Ferocity

Recently in about 1/10 rengar games, when I already have 4 Ferocity stacks and cast (and hit) e, my abiltys change to the Ferocity bonus abilitys (like it should) , but the 5Stack-E still have the CD of the normal E. I cant cast the 5-Stack E , till the cd is 0. I noticed this bug especially in the early game. I do not know if it depends on the items.