Ekko's stun doesn't work when someone dash

Hi, i don't know if this is a bug or not, but i'm maining ekko and there is something that is for me injust on him, when i stun someone ( with my w ) who has a dash ( like tryndamere E, urgot flip etc etc ) the animation isn't canceled and they are still doing the spell while being stun, isn't a " stun" supposed to block the ennemy even when he casting a spell ? Maybe it's normal and this is just me Thanks for reading