OK, opens as 16-bit for me. My suspicion: you once played with the file-tiff plugin, so you now have the old plugin somewhere, overriding the standard one.
Locate your Gimp profile (possibly in C:\Users\{your_id}\AppData\Roaming\GIMP\2.10 otherwise see the link above for ways to locate it) open the pluginrc file with a text editor and search for file-tiff-load. It should look like:
The important part is the ${gimp_plug_in_dir} which means that the file is loaded from Gimp's own plugin directory. It it loaded from somewhere else (typically, your the plug-ins subdirectory of your Gimp profile), then locate that file, erase/move it, and restart Gimp. You can also do the same for file-tiff-save. You don't need to edit the pluginrc file itself.
Locate your Gimp profile (possibly in C:\Users\{your_id}\AppData\Roaming\GIMP\2.10 otherwise see the link above for ways to locate it) open the pluginrc file with a text editor and search for file-tiff-load. It should look like:
Code:
(plug-in-def "${gimp_plug_in_dir}\\plug-ins\\file-tiff-load.exe" 1494435050The important part is the ${gimp_plug_in_dir} which means that the file is loaded from Gimp's own plugin directory. It it loaded from somewhere else (typically, your the plug-ins subdirectory of your Gimp profile), then locate that file, erase/move it, and restart Gimp. You can also do the same for file-tiff-save. You don't need to edit the pluginrc file itself.