When ACF adds data to the database it adds to entries in meta. The first is the meta key as you are inserting. The second is the ACF field key reference. The meta key for this field name prepended with an underscore "_{$field_name}" and the value of this is the field key of the field. Whithout this ACF does not know what to do with your field and is why it is not working and why it does work after you edit and update the user that was added.
