I must continue to misunderstand !
The 20 mm is poor in sharpness as it is a pancake !!
I have the sel24f1.8 and i find the 12 mm to be as sharp as the 24.
All review i read, and especially the ones coming from "le monde de la photo" and "chasseur d'image" which are famous specialized photography magazines in France, explains the Touit 12 mm to be sharper than the Touit 32 mm Touit. But you give huge different scores between the 32 and the 12 !!
So, may be you had a bad sample or it must be a mistake...