When I deleted the Finder .plist, all of the Finder Preference setting returned to default. The 'show file extensions' box is UNCHECKED by default.
There was no change to the folder with the Numbers files, still shows the extensions even though the Finder Preference is Unchecked and the Get Info 'hide extension' IS checked. (Emphasis added)
That's what I was wondering.
Something, somewhere, is corrupted.
Reinstalling Sierra or upgrading to High Sierra do, indeed, sound like your best options.
