BMPCC + rafcamera arri b to mft + optar 1.2/12mm = vignetting
I'm not familiar with the setup you described, but does that adapter have some sort of iris control built into it?  I have the adapter pictured below and it doesn't allow the BMPCC to adjust the aperture of the lens, which means I need to use the iris control to adjust the amount of light getting let into the camera.



The vignetting is caused by how the iris inside the adapter functions. When you close the iris, to restrict the light entering the camera, it also has the side-effect of creating a vignette.  This is the main reason I stopped using the BMPCC and went back to using the regular BMCC, because I don't have any MFT lenses.