When I use the built in image viewer to open a single tiff file which contains 2 images, Opus will only display one image and not offer an option to view the next image.
This is a multi image tiff, I.E. one tiff file, containing 2 images.
When I open with windows picture viewer, both images are accessible.
The Opus TIFF viewer (and many others) will only show the first image. If you want to view faxes (I presume that's what it is), you'll want to set another viewer for that file type. (You can keep the Opus viewer for other file types if desired.)
There is a plugin API for anyone who can write software and wants to add multi-page TIFF support, but demand for the feature seems to be very small these days, as the files themselves are very rare now.