Davidc502- wrt1200ac wrt1900acx wrt3200acm wrt32x builds

See this:

I experience this myself since my DHCP server is on another device. You'll be having this same cosmetic issue if you ping the IP address associated with the MAC address (handed out by the "Master" router) from the "Dumb" router (via LuCI or SSH) and suddenly get the ARP table populated (i.e. the question mark gets replaced with an IP address). Perhaps, if you really want to, you could write up a script that pings all (via ping/nmap/arping) of your LANs DHCP range on boot of the Dumb AP. Even then, it'll only show an IP address - you'll manually need to add the Static IP entries (and their respective host names) in the /etc/hosts file of the Dumb AP.