I know that there was a bug in the 2504 WLC code that matches this scenario, the WLC would always respond with the NAT IP address regardless of the CLI command issued. I wonder if you are hitting a similar deal? I don't have a 5508 available at the moment to test though.
CSCts52998
For that scenario it was fixed in 7.0.220.2 which is a special request via TAC.