Yesterday I started seeing random failures connecting with a new device, and after some troubleshooting realized that it had been given the address 192.168.80.218 even though it is not the device with the MAC address in the static-mac-ip-mapping. Unfortunately I've lost the console output from the `show ip dhcp-server binding 192.168.80.218` command, but it listed the same address twice, once with the new device's MAC address and once with the static-mac-ip-mapping address.
I then removed that address binding from the system, and rebooted the new device... and it got 192.168.80.217 assigned, which is still a duplicate. I then assumed that the pool must be 'full', and increased its size from 20 to 40 addresses, and removed the .217 binding. A reboot of the new device got a non-duplicate address, and now it works as it should.
From what I can tell, all of the non-statically-mapped addresses in the pool had active bindings, and so when a new device requested an address, the ICX gave it one of the statically-mapped address even though it should not have.