Thanks for that.
Release check says we're on " Tiki version 21.0" (freshly installed a few weeks back, following the official install docs for Debian).
The Classification "Categorise" button shows the correct category is selected.
The Cache settings *were* set, so I've cleared them, and clicked the "Clear Cache" button under Admin Modules. Unfortunately, that didn't work. I've also cleared the system caches.
Clearing the browser cache also didn't seem to work.
Any other suggestions?