Is there some kind of bug with heal affecting targeting?

So there's that little 5 second video. I just to make sure Illaoi stayed full on health in case people came to stop us from ending and when i do, my cursor changes and causes me to move instead of basic attack. I've had another instance of this in another game that didn't record but just assumed I messed up.

Is this a bug or just my own user error? I didn't move my cursor at all so I'm really confused about it.