Hi, I have only just seen this post, I have had the same issue with my 2016 M3W and mentioned it when I took took the car to the factory for some other warranty work. It was blamed on wear from getting in and out of the car. The leather was, I think, re-stained along the offending stitching.
I will be interested to see how they address your issue.

M3W, Bentley Continental 6.0 Flying Spur, MGB BGT. Beetle Conv, Merc Est, Renault Cleo.