Answering your points in reverse order:
Originally posted by Class A
If the lens produces gorgeous images -- and it does from what I've seen -- it does not really matter what may or may not be inferred from the MTF curves.
A fair point. the FA77 isn't apochromatic, at apertures of f/1.8 it suffers from visually distracting LoCa, PF, and lowered contrast -
but it is a superb lens, I own one.
Originally posted by Class A
divergence between the meridional and the saggittal planes looks like the textbook approach to achieving a flat field (leaving the saggittal plane slightly under corrected) so I'm not sure why you are suspecting the field to be not flat.
under-correcting those planes is
one approach to eliminating astigmatism, as is introducing some distortion to counteract the shift in the focal planes is another method. However, focus shift is a common symptom of under-correcting the focal planes, along with field curvature. As you mentioned the FA77 was designed in this fashion - but the FA77 is a wider lens, it also has FREE* incorporated into the optical design**
Originally posted by Class A
First, the 135/2.0 isn't a macro lens, and it is unlikely to be meant for flat field close up work.
It does use floating lens elements which is more common amongst close focus/macro lenses. While this lens may not be
explicitly called a macro lens, it appears to share features more common amongst the more popular 100mm+ macro lens designs.
*Fixed Rear Element Extension.
**to correct spherical aberration at close focusing distances.