ARAM - Aurelion Sol with Rylai's slows us with snowball

I just got off a game of ARAM where I had Syndra (rest of team was Vayne, Cait, Morgana, Jayce) and enemy had Aurelion Sol (rest Yi, Poppy, Ashe, Leona). Aurelion built Rylai's pretty early because we weren't doing very well. Soon I noticed that we were being slowed (even with the frost effect under us) even when not fighting. I realised it was Aurelion's snowball slowing us. I think this is not intended behavior. I didn't think of making a screenshot though, or checking the actual debuff, but there was definitely a debuff (once on me and once on Caitlyn) that slowed us with frost while not fighting.