You would need to add an acf/save_post filter https://www.advancedcustomfields.com/resources/acf-save_post/
In this filter you would test to see if you are saving a term in your taxonomy. If you are then you would get the field and call wp_set_post_terms() for each post in the relationship field https://codex.wordpress.org/Function_Reference/wp_set_post_terms
