non-www to www redirection issues

Question

Recently, I enforced non-www to www redirection on my website.

I am using these rules in my htaccess file:

Options -Indexes
Options +FollowSymLinks

<IfModule mod_expires.c>
  ExpiresActive On
  ExpiresDefault A1209600
  ExpiresByType text/html A1
</IfModule>

RewriteEngine On

RewriteCond %{HTTP_HOST} !^www\. [NC]
RewriteRule .* https://www.%{HTTP_HOST}%{REQUEST_URI} [L,R=301]

ErrorDocument 401 /error.php
ErrorDocument 403 /error.php
ErrorDocument 404 /error.php

It works well except that people coming form this link structure:

https://example.com/blog/9999/titlequerystring

get redirected that way by the server:

https://example.com/blog/9999/titlequerystring

then--

https://example.com/blog.php/9999/titlequerystring

then--

https://www.example.com/blog.php/9999/titlequerystring

The .php after "blog" brings users to error page no found.

It should redirect like that to work properly:

https://www.example.com/blog/9999/titlequerystring

This is a custom CMS. Not Wordpress. It uses Multiviews to work properly. My server is set with Apache 2.4+PHP 7.4 and PHP-FPM as handler.

I've been trying to fix this for a few days but I guess I am not skilled enough to diagnose it further.

Thanks.

As replied to CBroe, here is the htaccess file for mod_rewrite without Multiviews:

Options -Indexes
Options +FollowSymLinks

<IfModule mod_expires.c>
  ExpiresActive On
  ExpiresDefault A1209600
  ExpiresByType text/html A1
</IfModule>

<IfModule mod_rewrite.c>
    RewriteEngine On
    
    RewriteCond %{HTTP_HOST} !^www\. [NC]
    RewriteRule .* https://www.%{HTTP_HOST}%{REQUEST_URI} [L,R=301]

    RewriteCond %{REQUEST_FILENAME} !-f
    RewriteCond %{REQUEST_FILENAME} !-d
    RewriteRule .* loader.php [L,QSA]
</IfModule>

ErrorDocument 401 /error.php
ErrorDocument 403 /error.php
ErrorDocument 404 /error.php

loader.php: https://controlc.com/47512763