You mean clearing cookies in your browser? Or are you referring to some setting on either the router or the AP?
I'm curious about this as well, because I have a similar issue with the IP's of several devices connected to one AC66U that I use as a "Repeater" and with another AC66U used as a Media Bridge. Both AC66's connect with my main AC3200 router, on which I'm now running Merlin's 380.57.
The GUI on the AC3200 shows IP addresses for both AC66's, but when it comes to the IP's of individual clients attached to those devices, it will aggregate the IP addresses for those connected to the Repeater, showing a small reference number and pop-up text (e.g., "There are 5 devices connecting to the AC3200 through this device"). but it will only show the IP for one of several device's connected to the Repeater, and this is seemingly done at random. For example, it may one day show the IP address of an HP printer connected wirelessly to the Repeater, and then the next day, it will show the IP of a desktop PC connected to the Repeater via an Ethernet cable (but the HP printer is now not shown). There doesn't seem to be any rhyme or reason for this. The DHCP lease list in "Administration" also shows none of the device IP's or MAC's that are connected to the Repeater.
Just the opposite is true of the devices connected to the Media Bridge. Those are shown as though they are connected directly via wired connection to the AC3200 (even though the Media Bridge is, obviously connected wirelessly to the AC3200).
And you're saying that deleting cookies in your browser allowed all IP's connected to your AP to be visible? Hmmm. I'll try it, but I'm skeptical.