‘orderby’ is set by ACF after the acf/fields/relationship/query/ filter. The reason for this is that ACF groups the posts based on post type when posts can be selected from multiple post types. It also orders them by parent/child. There isn’t a filter in ACF that can be used to alter this.
To accomplish this you would have to add a pre_get_posts action inside of your filter that would be used by the query that ACF eventually runs. Altering the order at this point will break the relationship field display in some cases, especially when posts can be selected from more than one post type.
