Hello, I noticed this just today. Scanning negatives with the Xsane plugin for Gimp on Linux causes 16bits scans to be downsized to 8bit ones. Using the stand-alone version of Xsane does the job better and delivers 16bit tiffs (if Xsane is instructed to do 16bit scans of course).
I always thought that my (quite old but still good) scanner wasn’t able to do 16bit, but it is Gimp or the Xsane plugin who decides that 8bit is enough!
So do not scan from within Gimp if you want to work with 16bit files.