code to force server-side Gutenberg block to reload on a change to a Tax

block.js

/**
 * BLOCK: serverside block that changes on page tax
 *
 * Registering a basic block with Gutenberg.
 * Simple block, renders and saves the same content without any interactivity.
 */

//  Import CSS.
import './editor.scss';
import './style.scss';

const {__} = wp.i18n; // Import __() from wp.i18n
const {registerBlockType} = wp.blocks; // Import registerBlockType() from wp.blocks
const {ServerSideRender, Spinner, SelectControl} = wp.components;
const {withSelect} = wp.data;

/**
 * Register: aa Gutenberg Block.
 *
 * Registers a new block provided a unique name and an object defining its
 * behavior. Once registered, the block is made editor as an option to any
 * editor interface where blocks are implemented.
 *
 * @link https://wordpress.org/gutenberg/handbook/block-api/
 * @param  {string}   name     Block name.
 * @param  {Object}   settings Block settings.
 * @return {?WPBlock}          The block, if it has been successfully
 *                             registered; otherwise `undefined`.
 */
registerBlockType('example/block-server-side', {
	// Block name. Block names must be string that contains a namespace prefix. Example: my-plugin/my-custom-block.
	title: __('serverside block that changes on page tax'), // Block title.
	icon: 'admin-site-alt', // Block icon from Dashicons → https://developer.wordpress.org/resource/dashicons/.
	category: 'common', // Block category — Group blocks together based on common traits E.g. common, formatting, layout widgets, embed.
	keywords: [
		__('serverside'),
		__('dynamic'),
	],


	/**
	 * The edit function describes the structure of your block in the context of the editor.
	 * This represents what the editor will render when the block is used.
	 *
	 * The "edit" property must be a valid function.
	 *
	 * @link https://wordpress.org/gutenberg/handbook/block-api/block-edit-save/
	 *
	 * @param {Object} props Props.
	 * @returns {Mixed} JSX Component.
	 */
	edit: withSelect( select => {
		return {
      // this the magic this code listens to tax in cause a custom tax called resorts the deafult is 'tags' 
      tax: wp.data.select( 'core/editor' ).getEditedPostAttribute( 'resorts' )

		};
	} ) ( (props) => {
		const {
			className, tax
		} = props;

    // if we have no tax set for the page then just show a messege to save a call to server side
		if ( 0 === tax.length ) {
			return <p>{ __( 'Select the resort in the sidebar, 'mvc' ) }</p>;
		}
    // the server side block with the tax object getting passed                   
		return (
				<div className={className}>
					<ServerSideRender block="mvc/block-mvc-landing-pages-title"
									  attributes={ { tax: tax } }
					/>
				</div>
		);
	}),

	/**
	 * The save function defines the way in which the different attributes should be combined
	 * into the final markup, which is then serialized by Gutenberg into post_content.
	 *
	 * The "save" property must be specified and must be a valid function.
	 *
	 * @link https://wordpress.org/gutenberg/handbook/block-api/block-edit-save/
	 *
	 * @param {Object} props Props.
	 * @returns {Mixed} JSX Frontend HTML.
	 */

	save: (props) => {
		return null;
	},

});

class-render.php

<?php

namespace example;

class Render {
	public function __construct() {
		add_action( 'plugins_loaded', [ $this, 'register_dynamic_block' ] );
	}


	/**
	 * Register the dynamic block.
	 *
	 * @return void
	 * @since 2.1.0
	 *
	 */
	public function register_dynamic_block() {
		// Only load if Gutenberg is available.
		if ( ! function_exists( 'register_block_type' ) ) {
			return;
		}
		// Hook server side rendering into render callback
		register_block_type( 'mvc/block-mvc-landing-pages-title',
			[
      // add the tax to the allowed attributes in this case we got an array
				'attributes' => [
					'tax' => [
						'type'    => 'array', 
					],
				],

				'render_callback' => [ $this, 'render_dynamic_block' ],
			]
		);
	}

	/**
	 * Server rendering for mvc/block-mvc-landing-pages-title
	 */
	public function render_dynamic_block( $att ) {
    
    // defaults return values
		$resort_name   = 'No resort set';
		$location_name = 'No location set';

    // did we get a tax passed to us if lets use it
		if ( isset( $att['tax'] ) ) {
      // the object we get from editor is/maybe an array you have to check it out to workout what to extract
    	$terms[] = get_term_by( 'term_id', $att['tax'][0], 'resorts' );
		} else {
      // if not then lets terms for the page and render that 
			$terms = get_the_terms( get_the_ID(), 'resorts' );
		}

		If ( false !== $terms ) {

      // get the term meta and set the loaction
			$tax_meta = get_term_meta( $terms[0]->term_id );
			if ( ! empty( $tax_meta ) && isset( $tax_meta['location'] ) ) {
				$location_name = $tax_meta['location'][0];
			}
      // set the resort name
			$resort_name = $terms[0]->name;
		}

		return sprintf( '<div class="resort-title"><h1>%s</h1><h2>%s</h2></div>',
			esc_html( $resort_name ),
			esc_html( $location_name )
		);
	}
}

the working code after the help I got in this ticket
WordPress/gutenberg#19486