Do you have any DHCP configuration in your IAP? When you say "it hands out", are you referring to a DHCP server within your network? Is this the same DHCP server which is giving a management address for the IAP, overriding your static assignment?
When you view the configuration of the clients with the problem, do they have the statically set DHCP server in their configuraiton or is it the rogue one?
Have you investigated your LAN to determine what this rogue DHCP server is? Try running a Wireshark capture to view the DHCP Discover packets.