Finder not showing some tagged files and folders under Tags

I was finding that all files and folders under a specific folder on an external drive were not showing under their Tags in the Finder sidebar.

The thing that solved it for me was moving everything from the top level folder for which Tags were not working into a new folder (in my case a Dropbox folder, but others have had similar problems with other scenarios).

Now all my tags are working again - yay!!

None of the usual fixes worked (see them in other posts), including the following:

• Adding them under System Preferences > Spotlight > Privacy, closing System Preferences and then reopening it (after 30 seconds);
• Trying under another user account (creating a temporary user);
• Resetting SMC and NVRAM (a.k.a. PRAM);
• Trying in Safe Mode; or
• Reinstalling macOS (without erasing Macintosh HD).