Back in June 2019 mjb87 reported router clients with no name and no MAC address. He/she did not report the firmware being used but did say the MAC prefix was 3A:9D:82.
I too have been seeing router clients with a blank name and blank MAC address (firmware 7.1.2).
Because I have DHCP logging enabled, I know that the MAC prefixes were aa:5e:4e and 1A:07:2B. I traced down the client device - it is running Android 10 and using the new default of a randomized MAC address.
In the screen shot above, the two blank clients are the same device, connected to different SSIDs. Each SSID is assigned to the same VLAN. One SSID is WPA2 Personal, the other is WPA2 Enterprise.
Why is the MAC address (and name) not displayed for these clients?