I can replicate that. The default tiff export is a multi-page tiff. If the bottom layer is transparent then that is what you see in a file viewer.
Turn that option off and the image is flattened much the same as exporting as a jpeg.
If you want to keep the tiff pages and Gimp will open a mult-page tiff as layers then make a new-from-visible and put that layer bottom of the stack, and that is what will show in a file viewer.