子域名docs.example.com的404错误页无法正常显示问题求助

大家好，我现在碰到一个子域名的问题想请教一下：
当我在docs.example.com这个子域名下输入一个不存在的页面地址（比如https://docs.example.com/faultypage），服务器会自动重定向到目录里的index.php文件，但这个index.php是空的，导致浏览器只会显示空白页，没法正常返回“页面未找到（404）”的提示。我自己临时在index.php里加了跳转代码，让它跳转到https://docs.example.com/404page.html，但总觉得这不是规范的解决办法，想问问有没有更合适的处理方式？

下面是我当前的配置文件内容：

文档目录下的.htaccess文件 #

# GZIP compression for text files: HTML, CSS, JS, Text, XML, fonts

<IfModule mod_deflate.c>

AddOutputFilterByType DEFLATE application/javascript

AddOutputFilterByType DEFLATE application/json

AddOutputFilterByType DEFLATE application/rss+xml

AddOutputFilterByType DEFLATE application/vnd.ms-fontobject

AddOutputFilterByType DEFLATE application/x-font

AddOutputFilterByType DEFLATE application/x-font-opentype

AddOutputFilterByType DEFLATE application/x-font-otf

AddOutputFilterByType DEFLATE application/x-font-truetype

AddOutputFilterByType DEFLATE application/x-font-ttf

AddOutputFilterByType DEFLATE application/x-javascript

AddOutputFilterByType DEFLATE application/xhtml+xml

AddOutputFilterByType DEFLATE application/xml

AddOutputFilterByType DEFLATE font/opentype

AddOutputFilterByType DEFLATE font/otf

AddOutputFilterByType DEFLATE font/ttf

AddOutputFilterByType DEFLATE image/svg+xml

AddOutputFilterByType DEFLATE image/x-icon

AddOutputFilterByType DEFLATE text/css

AddOutputFilterByType DEFLATE text/html

AddOutputFilterByType DEFLATE text/javascript

AddOutputFilterByType DEFLATE text/plain

AddOutputFilterByType DEFLATE text/xml

</IfModule>

docs.conf虚拟主机配置文件 #

<VirtualHost *:80>

ServerAdmin office+web@example.com

ServerName docs.example.com

ServerAlias docs.example.com

DocumentRoot /var/www/brand/Documentation/new

RewriteEngine on

RewriteCond %{SERVER_NAME} =docs.example.com

RewriteRule ^ https://%{SERVER_NAME}%{REQUEST_URI} [END,NE,R=permanent]

</VirtualHost>

备注：内容来源于stack exchange，提问作者AZAZEL