Problem with register_rest_route call since 5.28.0: missing the required permission_callback argument
Don't panic, the issue you are seeing is only a notice, not an error and won't stop CiviCRM working.
In WordPress 5.5, a change was made to how REST routes are registered and now required a permission_callback. For the moment, the plugin will continue to work and once the plugin author releases an update that adds the permission_callback, the notice will disappear.
The reason you may see it the notice on one site and not all is that notices only appear when WordPress is run in DEBUG mode, confirm by checking the value of
wp-config.php for
define( 'WP_DEBUG', true );

If the site showing the notices should not be in debug mode, simply change the value of WP_DEBUG to false.