[Gameplay Bug. Gangplank and Sion ult conflict]

While playing a game I was fighting as vs , he ulted in my area, and in return I ulted towards him. Sion's ult as you all know is supposedly unstoppable and unslow-able, yet my running speed was reduced down all the way as though i were hit with his ult walking normally. This happened twice, but I am not sure if it matters whether I ult first or gangplank, or if I ult outside of his ult area and then run into his ult. This is a bug I noticed and I hope it gets patched. Thank you.