The TIFF output always exports in full range. I tested this by choosing “Open->Other->Color Bars->100% PAL color bars” and then exporting. When I inspect the exported file, white pixels have a code of FFFFFF and black pixels have a code of 000000. Maybe you can show how you are testing and we can help debug it.
If you want 16bit output, you can add this line to the “Other” tab in the advanced export panel: “pix_fmt=rgb48”
I got 255 in paint. The tif should be ok on a flat export, but when i use ‘Brightness’ & ‘Hue/Color…’ filter the black levels sink to my likings and Utvideo catch it in export, but maybe i dont get that filter in my Tif.