MU-Plugin to temporary fix the problem with the list of authors on post edit page with Block Editor.

fix-authors-bug-5.7.php

<?php
/**
 * MU-Plugin to temporary fix the problem with the list of authors on post edit page with Block Editor.
 * With WP 5.7, user with the Editor role is unable to search across long list of authors in Author combobox.
 *
 * @package kagg-design
 */

/**
 * Fixes bug in WP 5.7 with author selection on post edit in admin.
 *
 * @param bool $jsonp_enabled Whether JSONP is enabled. Default true. We do not modify it.
 *
 * @return bool
 */
function vp_rest_jsonp_enabled_filter( $jsonp_enabled ) {
	// phpcs:disable WordPress.Security.NonceVerification.Recommended
	if (
		isset( $_GET['who'] ) && 'authors' === $_GET['who'] &&
		isset( $_GET['context'] ) && 'edit' === $_GET['context'] &&
		current_user_can( 'moderate_comments' ) // Editor role.
	) {
		// Unset parameter which causes the permission problem.
		unset( $_GET['context'] );
	}
	// phpcs:enable WordPress.Security.NonceVerification.Recommended

	return $jsonp_enabled;
}

add_filter( 'rest_jsonp_enabled', 'vp_rest_jsonp_enabled_filter' );