I just noticed that the ports are no longer listed when I select a switch.
This is the case with the S8-POE and the S16-POE with firmware version 1.2j.
The MAC addresses have been removed, so the fact that a white area can be seen there is correct 
And they are back again!
Is there anything interesting about the network layout of the site? i.e. is there a double-NAT or outbound firewall rules of any kind?
Yes, I have installed a Firewalla Purple SE router.
But the only things being blocked is the traffic from the guest VLAN to the rest of the network.
So single NAT? I can’t tell from the screenshot, are you using Safari by chance?
My ISP router is in bridge mode, so single NAT. I was using Safari at the time and did not try an other browser.
Ok, that’s a known issue that’s on our radar. One way to fix it is to clear cache but that only seems to work for 1-3 days from what other customers are reporting. Another way is to use Chrome.
OK, thank you for your time and effort. I will see the fix appear in a future update.
I get this result when I have my Google Chrome tabs at the usual 10 or more open. Have to shut down, reboot and go in fresh.
microsoft Edge seems to work but I don’t use edge by default.