Laravel route collision problem

routes∕web.php

<?php

use Illuminate\Support\Facades\Route;

/*
 * Routes:
 * second/fifth
 * third/sixth
 *
 * ./artisan route:list
 * +--------+----------+----------------------------+------+---------+--------------+
 * | Domain | Method   | URI                        | Name | Action  | Middleware   |
 * +--------+----------+----------------------------+------+---------+--------------+
 * |        | GET|HEAD | api/user                   |      | Closure | api,auth:api |
 * |        | GET|HEAD | {group}/{page}             |      | Closure | web          |
 * |        | GET|HEAD | {group}/{page}/{optional?} |      | Closure | web          |
 * +--------+----------+----------------------------+------+---------+--------------+
 */

Route::prefix('{group}')->where(['group' => 'first'])->group(function () {
    Route::get('{page}', function () {
        return 'This page will 404.';
    })->where(['page' => 'fourth']);
});

Route::prefix('{group}')->where(['group' => 'second'])->group(function () {
    Route::get('{page}', function (string $group, string $page) {
        return "This is the {$page} page in the {$group} group.<br>In a larger project, this route would go to Controller2.";
    })->where(['page' => 'fifth']);
});

Route::prefix('{group}')->where(['group' => 'third'])->group(function () {
    Route::get('{page}/{optional?}', function (string $group, string $page) {
        return "This is the {$page} page in the {$group} group.<br>In a larger project, this route would go to Controller3.";
    })->where(['page' => 'sixth', 'optional' => 'A string no one will guess']);
});