Well, yes, there is that, the solution is not perfect and does not scale well. Another solution would be.
use an acf/save_post filter. When your posts are saved get the title of the post object field and store that in another custom field not controlled by ACF. Then sort based on this field. What this does is it makes the title of the related post a value associated with the post.
A more complicated solution would be to use $wpdb and construct your own MySQL query to do the work, but this solution is beyond me.
