Opus doesn't understand 128 bpp TIFF images, so the viewer can't display them.
The preview pane will ask Windows if it can generate a thumbnail for things that Opus and its plugins can't understand, but the standalone viewer won't. (Something we may revisit one day. When that was added, the shell was literally only good for small thumbnails, not full-size images which now come out of some of the thumbnailer components.)