为什么使用我的子域URL会显示不安全的HTTPS，而不是正常的域URL？

server {
    listen 80;
    server_name subdomain.yourdomain.com;
    return 301 https://$host$request_uri;
}

server {
    listen 443 ssl;
    server_name subdomain.yourdomain.com;

    ssl_certificate /path/to/your/certificate.crt;
    ssl_certificate_key /path/to/your/private.key;
    ssl_protocols TLSv1.2 TLSv1.3;
    ssl_ciphers 'ECDHE-ECDSA-AES256-GCM-SHA384:ECDHE-RSA-AES256-GCM-SHA384:ECDHE-ECDSA-CHACHA20-POLY1305:ECDHE-RSA-CHACHA20-POLY1305:ECDHE-ECDSA-AES128-GCM-SHA256:ECDHE-RSA-AES128-GCM-SHA256';
    ssl_prefer_server_ciphers on;

    location / {
        root /path/to/your/website;
        index index.html index.htm;
    }
}