Hierarchical select for native Advanced Custom Fields taxonomy fields.

custom-acf-admin.js

jQuery(document).ready(function($){

	//Modify select2 ajax request to include a parent_id parameter, used by custom_acf_taxonomy_hierarchy()
	acf.add_filter('select2_ajax_data', function( data, args, $input, field, instance ){

		var target_field_key = 'field_5c7634ca3413f'; //YOUR TARGET FIELD KEY HERE

		if(data.field_key == target_field_key){

			var parent_id = 0; //by default we want terms with parent of 0 (top level)
			
			if($input.val() != '' && $input.val() != null){ //if we have chosen one or more terms already
				var values = $input.val();
				parent_id = values.pop(); //get last selected term to use as the parent for the next query
			}
	
			data.parent = parent_id;
		}

	  return data;

	});
	
});

hierearchical-select-acf.php

<?php

//Modifies ajax requests from our select2 dropdown, $args = WP_Term_Query arguments
function custom_acf_taxonomy_hierarchy( $args, $field, $post_id ){
    //default to 0 (top-level terms only) unless we get a different parent with the request
    $args['parent'] = empty($_POST['parent']) ? 0 : $_POST['parent'];
    
    //To allow multiple top-level selections, we can go back to the top of the taxonomy tree after we reach the end of a branch
    /*
    if(!empty($args['parent'])){
        $child_terms = get_term_children($args['parent'],'YOUR_TAXONOMY_SLUG'); //change to your taxonomy slug
        if(empty($child_terms) || is_wp_error($child_terms)) $args['parent'] = 0;
    }
    */
    
    return $args;
}
add_filter('acf/fields/taxonomy/query/key=field_5c7634ca3413f', 'custom_acf_taxonomy_hierarchy',10,3);


//Make the javascript available in the admin when editing a post with ACF
function custom_admin_enqueue_scripts(){        
    wp_enqueue_script('custom-acf-admin-js', get_stylesheet_directory_uri() . '/js/custom-acf-admin.js',['jquery']);
}
add_action('acf/input/admin_enqueue_scripts','custom_admin_enqueue_scripts');

albertvisuals

FOund some workaround/fix?