跨域问题,纯后端,前端访问的时候可能跨域

问题描述:我有一个接口,偶尔跨域,偶尔不跨域,为什么会出现这个情况?
测试猜测:
1.请求没请求过的合同,会出现跨域,在请求可能不会
2.多次频繁的访问接口.也有一次可能不会出现跨域
注:不跨域后在请求就不会跨域了

App\Http\Kernel文件下的protected $middleware加了
\App\Http\Middleware\EnableCrossRequestMiddleware::class,

<?php

namespace App\Http\Middleware;

use Closure;
use Illuminate\Http\Response;

/**
 * 跨域中间件
 */
class EnableCrossRequestMiddleware
{
    /**
     * Handle an incoming request.
     *
     * @param  \Illuminate\Http\Request  $request
     * @param  \Closure  $next
     * @return mixed
     */
    public function handle($request, Closure $next)
    {
        $response = $next($request);
        $header=[
            'Access-Control-Allow-Origin' => "*",//允许所有资源跨域
            'Access-Control-Allow-Headers' => 'Origin, Content-Type, Cookie, Accept, Authorization',
            'Access-Control-Allow-Methods' => 'GET, POST, PATCH, PUT, OPTIONS, DELETE, HEAD',//运行的请求方法
            'Access-Control-Allow-Credentials' => 'true'//运行客户端携带证书式访问
        ];

        if ($response instanceof Response) {
            foreach ($header as $key => $value) {
                $response->header($key, $value);
            }
            return $response;
        }

        foreach ($header as $key => $value) {
            $response->headers->set($key, $value);
        }

        return $response;
    }
}