Discovered in this beta 2, if you assign static IP in Network Map->Clients->Client status and you give it a name, it registers well on the DHCP server page, but it leaves the "name" blank.
That's a known limitation. That's because Asus's Networkmap lets you enter anything since it's used as a description, not just a valid hostname. Allowing these to be reused on the DHCP reservation would lead to a lot of invalid entries.