[GAMEPLAY] When Jayce uses his R to change stances, he is sometimes prevented from using abilities

Yesterday I was playing Jayce and pushing the first mid turret using my abilities to improve my damage against turrets, typical Jayce stuff, and I came across with a strange bug: I used my ultimate to switch to hammer form in order to use the passive bonus, but despite receiving the "passive ready" buff, switching the ability icons for the hammer ones and playing the animation and sounds (the lightinghs and magic thingies), he was still in the cannon stance, and, afterwards, when I tried using my abilities, nothing happened. Then I died, and when I respawned, everything was back to normal. But still, I died because of this. I don't know how I did it exactly but I used my W Hyper Charge and then pressed R as soon as I launched the last auto-attack I used my ultimate and then it happened. Sadly I have no images or videos because it happened very fast. BUT IT HAPPENED! Thanks in advance :)