Bug {game play}

While playing full attack damage Sion for fun , Enemy team had ADC Ashe , So i thought about charging towards her with my ultimate then -Q- -W- her instant death . But something weird for me happened 3 times in the same game . Ashe was able to time her ultimate to stun me as soon as i hit her with my ultimate . I think this shouldn't happen because Sion ultimate gives him immunity to all kinds of Crowd control until he stops it or hit enemy champion , and when he hit enemy champion he stun him for 0.75-1.75 seconds according to distance travel. I used me ultimate on Ashe 3 times 1 full range 1 half range 1 close range and she could stun me all the times . This makes no sense please fix this bug i don't care if that Ashe is pro and can time her ultimate so the player deserve it or any of that nonsense talk , I want Sion ultimate to work like it should , Immune until stop or hit then INSTANTLY STUN enemy champion it hit .