Probably the optical axis of the main lens is not coincident with the optical axis of the flattener. Might be a tilt / mis-collimation in the main lens.
I suppose it is also possible that the lenses in the flattener are slightly mis-aligned or tilted. Probably you could do some test like rotating the flattener 180 degrees, and see what happens to the images.
Spacing change might help, but this is not a simple spacing problem. Spacing error alone would make all the corners elongated together.
Are you using all threaded connections? Or are there slip-in 2" connections with the focuser? Wonder if the flattener + camera is tilted in the focuser...