I'm also having this issue. Any solutions or workarounds?
I used BatchFormatConversion to go from RAW to FITS, and this created the thumbnail. There doesn't seem to be an Output format hint to stop thumbnail generation.
(I know, FITS is deprecated, but I want to use it so I can open files later in Matlab/Octave. And would rather not have extra intermediate conversion steps from .NEF to .XISF to .FITS if I can help it.)
Thanks