Historically the Endpoint IP address was learnt via SNMP polling of the infrastructure ARP tables.
The Framed-IP-Address is learnt from the RADIUS Accounting - with ArubaAOS this comes in with the Start, most other equipment this comes in with the Interim. However, this is all dependent on the NAS ability to learn the device's IP - most NAS have the ability to track the DHCP exchange, static IP addresses are usually more problematic.
In about ClearPass v6.6 I believe they made the Endpoint's IP address = the Framed-IP-Address.
If the RADIUS Accounting is not enabled, or the Start/Interim are not enable, or the NAS is not learning the device's IP address then the Framed-IP-Address is not populated: You are dependent on the SNMP poll which by default occurs every hour - it can be tuned down to every 10 minutes...
The knock-on consequence of not getting the Framed-IP-Adddress is that upper-layer integration (post-auth profiles) that are reliant on this IP address (eg injecting into a firewall) will not occur.