Routes confict

App_routes.php

<?php
/**
 * App Routes configuration App/config/routes.php
 *
 * In this file, you set up routes to your controllers and their actions.
 * Routes are very important mechanism that allows you to freely connect
 * different URLs to chosen controllers and their actions (functions).
 *
 * CakePHP(tm) : Rapid Development Framework (http://cakephp.org)
 * Copyright (c) Cake Software Foundation, Inc. (http://cakefoundation.org)
 *
 * Licensed under The MIT License
 * For full copyright and license information, please see the LICENSE.txt
 * Redistributions of files must retain the above copyright notice.
 *
 * @copyright     Copyright (c) Cake Software Foundation, Inc. (http://cakefoundation.org)
 * @link          http://cakephp.org CakePHP(tm) Project
 * @license       http://www.opensource.org/licenses/mit-license.php MIT License
 */

use Cake\Core\Plugin;
use Cake\Routing\Router;

/**
 * The default class to use for all routes
 *
 * The following route classes are supplied with CakePHP and are appropriate
 * to set as the default:
 *
 * - Route
 * - InflectedRoute
 * - DashedRoute
 *
 * If no call is made to `Router::defaultRouteClass`, the class used is
 * `Route` (`Cake\Routing\Route\Route`)
 *
 * Note that `Route` does not do any inflections on URLs which will result in
 * inconsistently cased URLs when used with `:plugin`, `:controller` and
 * `:action` markers.
 *
 */
Router::defaultRouteClass('InflectedRoute');

Router::scope('/', function ($routes) {
/**
 * Here, we are connecting '/' (base path) to a controller called 'Pages',
 * its action called 'display', and we pass a param to select the view file
 * to use (in this case, src/Template/Pages/home.ctp)...
 */
	$routes->connect('/', ['controller' => 'Pages', 'action' => 'display', 'home']);

/**
 * ...and connect the rest of 'Pages' controller's URLs.
 */
	$routes->connect('/pages/*', ['controller' => 'Pages', 'action' => 'display']);


/**
 * Connect catchall routes for all controllers.
 *
 * Using the argument `InflectedRoute`, the `fallbacks` method is a shortcut for
 *    `$routes->connect('/:controller', ['action' => 'index'], ['routeClass' => 'InflectedRoute']);`
 *    `$routes->connect('/:controller/:action/*', [], ['routeClass' => 'InflectedRoute']);`
 *
 * Any route class can be used with this method, such as:
 * - DashedRoute
 * - InflectedRoute
 * - Route
 * - Or your own route class
 *
 * You can remove these routes once you've connected the
 * routes you want in your application.
 */
        //This route will work after uncomment
	//$routes->connect(
	//':slug',
	//['controller' => 'Posts', 'action' => 'view'],
	//['routeClass' => 'SlugRoute']
        //);
	$routes->fallbacks('InflectedRoute');
        //This route does not work after uncomment
	//$routes->connect(
	//':slug',
	//['controller' => 'Posts', 'action' => 'view'],
	//['routeClass' => 'SlugRoute']
        //);
});

/**
 * Load all plugin routes.  See the Plugin documentation on
 * how to customize the loading of plugin routes.
 */
Plugin::routes();

MyPlugin_routes.php

<?php
// routes for plugin MyPlugin

This plugin route does not work if app fallbacks called
Router::connect(
':slug',
['plugin' => 'MyPlugin', 'controller' => 'Posts', 'action' => 'view'],
['routeClass' => 'MyPlugin.SlugRoute']
);