WordPress site goes blank in random intervals

I’ve a WordPress site which 2 or 3 times a week rewrites the index.php and .htaccess files. After it happens the site goes blank, can’t open /wp-admin or anything else for that matter. Each time I have to replace the index.php and .htaccess files with the originals to pull the site back.

This is a fragment of the index.php after WordPress modifies it:

enter image description here

and this is the modified .htaccess:

<IfModule mod_rewrite.c> RewriteEngine On RewriteBase / RewriteRule ^(index|wp\-admin|wp\-include|wp\-comment|wp\-loader|wp\-corn\-sample|wp\-logln|output|about|admin|wp\-ver).php$   - [L] RewriteRule . index.php [L] </IfModule> 

I tried removing write privilege from these files, yet they keep being overwritten.

I tried disabling plugins, but it still happens. Can a removed plugin still affect the site?

I’m using a theme called Avada.

What could be causing this problem? Any help would be greatly appreciated.