This issue is documented in Cisco bug ID CSCsf22066.
This problem occurs when a Cisco Adaptive Security Appliance (ASA) is configured as a DHCP proxy with the dhcp-network-scope 172.16.32.0 command for the VPN Client DHCP IP address assignment. The DHCP server used by the ASA is an external server with the 172.16.32.0/24 range of addresses available. When the VPN Client connects to the ASA and the ASA attempts to download a DHCP IP address from the external DHCP server, the ASA presents the reply giaddr (DHCP proxy-server address) to the DHCP server as 172.16.32.0 instead of its actual physical interface address. This behavior was first found in version 7.1(2).
The only workaround for this issue is to use an internal ASA DHCP range.