Does is show those assignments in the GUI list along with the others?
I just tried on my RT-AC68U. The IP and hostname assignment was made for the device, and the device shows in the Client List (as expected), but the client doesn't show in the LAN/DHCP Server list. Makes sense that if doesn't show there. I plan to just manage the list in a Google Sheet with a simple formula to build the conf file line, then I can just cut and paste the lines into the conf file.
=CONCATENATE("dhcp-host=",A2,",",B2,",",C2)
Also, you can see your active leases at: cat /var/lib/misc/dnsmasq.leases
(I think this is the correct path)
In the Network Map/Client List, the devices defined in the DHCP conf file show with their defined name, but it took a minute before the host name showed up. Not all the devices in the client list are showing their defined host names, but maybe they have to renew their lease.
Finally, I'm not sure the ",12h" is needed at the end of the line above. I think you only use that if you don't want to use the default lease time.
Update: I installed
YazDHCP from amtm (after installing entware) and woohoo, the GUI DCHP can now take up to >200 entries. Perfect for me. Also, I can still manage the list on a sheet and perform a mass update if I have to.