This 21mm Biogon is a symmetrical design and in fact Zeiss does not even recommend its use with the M9, due to the corner discoloration. If you are getting this issue even with the A7S, which is probably the sensor most likely to work with wide angle rf lenses, then it is not likely the A7r ll can show any improvement. However, if this is purely a corner discolouration problem, you can try Cornerfix or Adobe Flat Field plug-in to remove this effect. In the case of my the Leica 21mm Elmarit-Asph, the issue is corner and edge smearing. This cannot be corrected by using software. Leica had to use a thinner than usual UV/IR filter over the sensor to eliminate this smearing problem. But with the M8 the glass was too thin and therefore the IR filter was not effective enough to block all the IR. This gave rise to the infamous magenta cast with certain black synthetic materials.