Thank you Nik, for the reply. Just a couple of points;
1. There is consistency in that exported files that have no Finder thumbnail always have no thumbnail after repeated exports, and those that behave, consistently export with Finder thumbnails.
2. The problem appears with HEIF10; but both JPEG and HEIF8 export with Finder thumbnails.
3. All the files in Nitro are from scanned B&W prints and stored as 16bit greyscale TIFFs from which I am exporting processed versions.
(I have come back to this today, after rebooting my Mac and I am seeing what I saw yesterday… )
Best wishes,
John Perry