I agree - I think it's a settings issue in Silverfast, where the defaults are being confused by the color of the stain. If you check curves in Silverfast, my guess is that the curve is back-tracking on itself, causing the posterization or near-solarized appearance.
I usually scan as 48-bit RGB, rather than greyscale, but sometimes have to scan as a positive, and then invert in Photoshop. Part of that trick is to remember that the settings values in Silverfast are reversed. I also ignore the Epson instructions for my 3200 to scan emulsion side up. I put the emulsion side down, as you would in an enlarger, and then flip the image in PS. Much sharper that way (no surprise, eh?).