[action_user] is just the user id that originally fired the auto action. It is not meant to be looped. You can access the looped user with loop_ substitutions. So for example if you need the user id you can use [loop_user_id]. This is noted in the description of the Parameters > Loop parameter. If you mean you're just acting on an array result from User then you wouldn't use [action_user] at all and would just use [user_id].