@Wayne-TP please see my answers below...

Is your phone on the local network when checking the Tapo app?

Yes my phone was connected to my TP Link router (however I suspect as soon as the Tapo app is launched it will request an internet connection which will then use mobile data - or can you confirm that the Tapo app code is written to only route via WAN if a LAN connection is not available?)

Please verify whether the WiFi smart devices are still connected to the WiFi when WAN connection is down.

I can confirm that my Tapo cameras were still connected when WAN was down as they were still viewable locally via my TP Link Vigi NVR on my TV screen.  Regarding smart plugs, smart buttons and other devices hard to tell as they simply all stopped working (buttons became unresponsive and all devices were showing offline in the app) but the fact the cameras remained online and connected to the Vigi NVR proves the LAN was available to the other smart devices via WiFi.

There are two types of automation rules: local smart rules continue to operate under the hub's control when the internet is down, while cloud-based rules do not function without an internet connection. Do you have any local smart rules? What is Local Smart Action?

I have a number of smart buttons which are connected to 2 smart hubs and control smart plugs via basic smart actions (ie turn lamps on and off).  These automations I presume are local and not "cloud-based rules" as you put it but the terminology you use here does not actually feature in the Tapo app set-up screens so I can't be sure.

What I am sure about is that it makes no sense that a button in my living room won't turn on the lights in my living room just because my internet connection is down. The button, the hub and the smart plug are all in the same room and simply do not need any cloud based rules or the internet to talk to each other do they?  The routing here should be local and not bouncing off the cloud.

The Vigi NVR proved that 6 Tapo cameras were still connected via LAN when the WAN was unavialable whereas the Tapo app showed the exact same devices as offline.  It appears the Tapo app was unable to connect via LAN and by default just gave up because there was no internet.  Please confirm how this code is written - if as I suspect it always needs an internet connection then we have a bug to resolve.  If the app should route via LAN when no WAN is available then hopefully you can help me find the setting(s) that may be preventing it from operating as intended.

Thank you in anticipation.