I don’t know if this will help anyone. A while back I created my own plugin that does this automatically when updating relationship fields. It was something that I needed for a client. Relationships between the same post type work automatically and relationships between different post types are accomplished by adding a field with the same field name to both post types. The only requirement is that they need to be top level fields, i.e. not a repeater or flex sub field. https://github.com/Hube2/acf-post2post
