replace CMB2 register_rest_field handlers with our own

cmb2-register-reset-field.php

<?php

/**
 * Registers a new post type
 * @uses $wp_post_types Inserts new post type object into the list
 *
 * @param string  Post type key, must not exceed 20 characters
 * @param array|string  See optional args description above.
 * @return object|WP_Error the registered post type object, or an error object
 */
function jt_register_test_cpts() {
	$labels = array(
		'name'               => __( 'BLahs', 'text-domain' ),
		'singular_name'      => __( 'BLah', 'text-domain' ),
		'add_new'            => _x( 'Add New BLah', 'text-domain', 'text-domain' ),
		'add_new_item'       => __( 'Add New BLah', 'text-domain' ),
		'edit_item'          => __( 'Edit BLah', 'text-domain' ),
		'new_item'           => __( 'New BLah', 'text-domain' ),
		'view_item'          => __( 'View BLah', 'text-domain' ),
		'search_items'       => __( 'Search BLahs', 'text-domain' ),
		'not_found'          => __( 'No BLahs found', 'text-domain' ),
		'not_found_in_trash' => __( 'No BLahs found in Trash', 'text-domain' ),
		'parent_item_colon'  => __( 'Parent BLah:', 'text-domain' ),
		'menu_name'          => __( 'BLahs', 'text-domain' ),
	);

	$args = array(
		'labels'              => $labels,
		'hierarchical'        => true,
		'public'              => true,
		'show_ui'             => true,
		'show_in_menu'        => true,
		'show_in_admin_bar'   => true,
		'menu_position'       => null,
		'menu_icon'           => 'dashicons-universal-access',
		'show_in_nav_menus'   => true,
		'publicly_queryable'  => true,
		'exclude_from_search' => false,
		'has_archive'         => true,
		'query_var'           => true,
		'can_export'          => true,
		'rewrite'             => true,
		'show_in_rest'        => true,
		'capability_type'     => 'post',
		'supports'            => array( 'title', 'thumbnail', ),
	);

	register_post_type( 'blah', $args );
}
add_action( 'init', 'jt_register_test_cpts' );

/**
 * Hook in and add a demo metabox. Can only happen on the 'cmb2_init' hook.
 */
function jt_register_test_cpts_mb() {
	$cmb_demo = new_cmb2_box( array(
		'id'                     => 'blah_metabox',
		'title'                  => __( 'BLah Metabox', 'cmb2' ),
		'object_types'           => array( 'blah', ), // Post type
		'show_in_rest'           => WP_REST_Server::ALLMETHODS,
		'register_rest_field_cb' => 'jt_register_rest_field_cb',
	) );

	$cmb_demo->add_field( array(
		'name'       => __( 'BLah URL', 'cmb2' ),
		'id'         => 'blah_url',
		'type'       => 'text_url',
		'repeatable' => true,
	) );
}
add_action( 'cmb2_init', 'jt_register_test_cpts_mb' );


function jt_register_rest_field_cb( $rest_box ) {
	register_rest_field( 'blah', 'pizza', array(
		'get_callback' => function( $object, $field_name, $request, $object_type ) use ( $rest_box ) {

			if (
				'pizza' === $field_name
				&& 'blah' === $object_type
				&& isset( $object['id'] )
			) {

				$values           = array();
				$main_object_type = 'post';
				$result           = CMB2_REST::get_box_rest_values( $rest_box, $object['id'], $main_object_type );
				if ( ! empty( $result ) ) {
					$values[ $rest_box->cmb->cmb_id ] = $result;
				}

				return $values;
			}
		},
		'update_callback' => function( $values, $object, $field_name, $request, $object_type ) use ( $rest_box ) {
			$main_object_type = 'post';

			if (
				'pizza' === $field_name
				&& 'blah' === $object_type
				&& ! empty( $values )
				&& is_array( $values )
				&& ( $object_id = CMB2_REST::get_object_id( $object, $main_object_type ) )
			) {

				$updated = array();
				$result  = CMB2_REST::santize_box_rest_values( $values, $rest_box, $object_id, $main_object_type );
				if ( ! empty( $result ) ) {
					$updated[ $rest_box->cmb->cmb_id ] = $result;
				}

				return $updated;
			}
		},
		'schema' => null,
	) );
}