Which version of Gimp are you using ? If the version is before Gimp 2.10.30 then it might be the embedded thumbnail causing the problem. It is a Windows problem.
Try opening the tiff image, then export with embedded thumbnail option OFF.
If that is the problem recommend updating Gimp.
Edit: If you export the tiff without All layers checked then that is equivalent to flattening the image. Just single layer, same as a png or jpeg.
Try opening the tiff image, then export with embedded thumbnail option OFF.
If that is the problem recommend updating Gimp.
Edit: If you export the tiff without All layers checked then that is equivalent to flattening the image. Just single layer, same as a png or jpeg.