This issue resolved when my Nest WiFi router went offline for 30-minutes and forced the two Nest Hub Display devices to resync with all devices when the WiFi network came back online (I am speculating - I have no logs to validate and only saw the Google Nest Hub Displays taking long time re-sync'ing before returning to their normal operation). When the Internet access resumed my iPhone's Google Home app was showing all the Tapo group names and Google Assistant everywhere was able to recognize those group names.

So, in summary, the original issue appears to be on Google/Nest side, and it was nothing to do with Tapo devices or services.