thank you for the very interesting reply! I was previously unaware of how correcting one aberration of a lens negatively affects others.
You are right that distortion control of the Loxia is excellent, this is one reason why I chose it over the Sony FE 35/2.8, which I also considered. I should say that generally I am very happy with the Loxia, and the field curvature issue is certainly not a deal-breaker for me. Especially as the effect is only noticeable in some pictures and I think it is more or less pronounced depending on focus distance. I will examine this further and also try f/11 and smaller apertures.
Closely examining some full-resolution photos I found online taken with the Sony FE 35mm lens it indeed looks to me like the Sony shows a similar field curvature as the Loxia. I have to confess I have not read your review of the Distagon ZM, but will do so now. 🙂
Best regards, Arthur