It is ordering by the post ID of the post object selected… unless these post object fields allow multiple values, in this case it is ordering by an a serialized array value.
There isn’t a way to order posts alphabetically by the title of a the post selected in a post object field. The title belongs to the post selected and not to the post you are trying to order. In order to do this the post title of the other post must be a meta value associated with the post you are trying to order. You need to create an acf/save_post filter, get the value of the post object field, get the title of the post object and store that value in another meta field and then use this meta field to sort the posts.
