Custom 404 shows "File not found" for php files instead of redirecting
I suspect the difference is that php files launch the interpreter while html files are just served. A solution that works but might not be the only one or the best one is to add this directive to the host configuration file:
ProxyErrorOverride on
This happens because you might've enabled the mod_proxy_fcgi and you are using PHP-FPM. The default error page in the proxy is File not found. So add the following to your httpd.conf or apache2.conf, not to your .htaccess.
It makes your .htaccess file's ErrorDocument directive to override the default Proxy error page.
ProxyErrorOverride OnThanks.