Are you using a center filter?
I noticed light fall off and greener edges when doing stitched images with a 35mm Apo Sironar Digital and sliding adapter.
I used before, but the problem is that it suck up 3 whole stops. It might be not a problem with film but with a 25 ISO digital back is a real pain when shooting at at iso 3. I get way better result shooting without it then applying the right ICC profile for each image in Leaf Capture. Is a bit of work create all the profiles, but once u save them is just a click.
And it take away that green cast. Here a sample of corrected image.