Drupal base_url incorrectly writing extra subdomain We have a basic drupal multi-site installation on a RHEL (RedHat) server.
For some reason when viewing the site, the asset URLs (CSS/JS) all point to the wrong subdomain.

Our site URL: http://drupal.sandbox.domain.com the drupal index page loads up with asset URLs being written as: http://drupal.sandbox.sandbox.domain.com.
Why would drupal add in the extra .sandbox subdomain?

The .htaccess file has been left as the default, also attempted to set RewriteBase to /, but there was no change. The sites/default/settings.php file has the base_url set to: $base_url = 'http://drupal.sandbox.domain.com' but Drupal is overwriting this somewhere. Setting the $base_url to the IP of the server causes the assets to be linked to the correct location (using the IP rather than the actual domain), but this is not ideal.

I've tested using the same configuration on an Ubuntu Server without any problems. I'm not sure exactly how to track down the cause of this problem.

How would I go about debugging this and discovering what is causing the overwritten URLs?
This is not a Drupal-specific problem. This can easily be checked by creating a standalone PHP page and using the same code to test. Being that the variable is overwritten immediately after it is set shows that PHP/Apache is the one doing the overwriting.

It is possible for Apache to overwrite input and output. See: What would cause PHP variables to be rewritten by the server?