You should verify that your CAS configuration is set to mark the connector as "secure". Assuming I have the right resource, the configuration documentation could be here: https://apereo.github.io/cas/6.3.x/configuration/Configuration-Properties.html#http-proxying
Also, this is a few years old but it might apply to your situation: https://fawnoos.com/2018/01/05/cas-deployment-with-proxy/
Hope this helps...
Hi! We managed to solve the issue and hide the server behind F5.
We didn't need to specify the connector as "secure". It was prooved that the CAS wasn't containing the correct URL to match the F5. The CAS configuration needed to be updated on both application parameter & application definition in CAS. Thank you for your support.