Lulu cancels her auto attack when used E on High atk spd ally

I think it is weird, I have tried a lot of times . When a ally is near me and I use my shield on ally, pix follows ally's attack. However, I find that if that ally has a relatively high attack speed, and i cannot auto attack. It is more like lulu cancels her attack animation automatically. ( I was standing and let her free auto, however I cannot fire any thing as it cancelled my attack, and the target was still exist, not killed by others before i attack)