Drupal 6 CAS not working as a CAS client to CAS Server that is validating service URLs

Hello,

We have used Drupal 6 CAS to make our Drupal site a CAS client to a CAS 2.X server (the version of our CAS server is old because it is a builtin CAS server with our Luminis portal).

We have configured our Luminis CAS server to validate service URLs. We have already got one of our other CAS client apps working. Our Drupal site is not working properly because it seems like the ServiceURL is different depending on what CAS protected page we are loading. This is how we are using our Drupal site CAS:

1) Logging in via CAS on the /user login page
2) Linking from our Luminis portal to different CAS protected URLs that asks the user to login to the CAS server if they are not logged in already. If a user is logged into Luminis already then they go straight to the page that is linked to but if they are not logged into Luminis (like if I share a direct link for example) then the user is asked to login to Luminis CAS before being allowed to open the link.

In the CAS log we see this:
phpCAS::setFixedServiceURL('https://www.hostname.com/cas?destination=user') [cas.module:284]

We notice that this service URL is different depending on what CAS protected page we try to access. What are our options here? I read a post from 2009 where a user hardcoded the service URL so it would not change for different pages so the service URL matched what the CAS server expected. Not sure this is really the best way to go and I am surprised I cannot find much discussion about this so I am thinking we are doing something wrong.