Marco, your sample seems to behave similarly to my M. Zuiko 25/1.8 for m4/3. Its image is soft on the left side until around f5.0, which improves at f5.6 and further.
As you already know, the culprit is the field curvature. And, according to the recent test blog of LensRentals, the curve is not perfectly level.
My fist sample of 25/1.8 bought new showed the affect of the curvature very badly, and Olympus replaced it with a noticeably better sample, but not perfect.
Like you, I didn't want to continue replacing it in search of the impeccable sample and unwillingly decided to stay with my second sample.
