thats a current bug with a lot of champions that have auto attack modifiers. there was a thread about it a week or two ago that literally had a list of about 15+ champions known to be effected by it due to their kits interaction with auto attacks.
its something in the code that makes champs with attack modifiers lose their target and just revert to standing. dont remember if riot mentioned they knew about it (though im sure they do) and im sure i havent seen them mention any fix on it (meaning they clearly havent found the problem yet.)
which happens. Shen once had a bug that randomly stopped his ult from executing after completing the channel for over a year before it was found. Riot bragged about it after they finally found it.