AnyConnect posture discovery uses same method as CWA (Central WebAuth) for finding the PSN. For test when in redirected state, if you open web browser do you get redirected to client provisioning page? If this simple test fails then AnyConnect will not be able to find the PSN as well.
Alternatively, ISE 2.2 & AC4.4 can be configured to find PSN without relying on URL-redirect. See following link for more information:
https://www.cisco.com/c/en/us/support/docs/security/identity-services-engine-22/210523-ISE-posture-style-comparison-for-pre-and.html