I'm not sure what was going on, but here's the sequence of events:
Phone was previously registered on another domain
I changed my DHCP option 66 to move the phone to a different domain
I did a factory reset on the phone
When the phone rebooted, it created a mac entry in devices in the old domain - it was as if the factory reset didn't quite work and the DHCP option was ignored, but it wasn't - the phone had the correct entry for the domain
I cycled through this (delete mac entry in old domain, factory reset the phone) a few times with the same result - it always created a mac entry in the old domain
I finally manually entered the mac address in devices in the new domain and the phone registered just fine
I then deleted the mac in devices in the new domain, did a factory reset on the phone, and when it rebooted, it correctly created the device entry in the new domain
For some reason, there was some remnants of the previous mac address "somewhere" in fusion that was pointing to the old domain and it didn't want to let go.