[Gameplay] Lulu's Polymorph doesn't stop Kalista from Auto-attacking

Playing a game as Lulu just now against a Kalista. When I went to polymorph her, I noticed she was still doing her leap animation. I wasn't sure as to why until the second time, when I realized she was still damaging me and applying stacks. I am not sure if this is because she can't cancel her auto attacks, or if it is a bug on Lulu. However it was noticed and I thought I should post it here. Is this supposed to happen or is it a bug?