... and discovered a somehow similar mark. I do have the same mark, but only about 1/3 circle. Under the loupe I can tell that this is not on the dial but appear to be on the inner face of the crystal !!!???!!!
My most plausible hypothesis (although very surprising) would be that the tip of the minute hand is (or has been) in contact with the crystal and left a scratch or a mark on the coating. That sounds weird but the spacing between the top hand and the crystal should be very small on those ultra thin. I'm now wondering if it is a known issue at JLC, present on a wider scale or just an isolated problem. Maybe Nico would know who to ask...
I'm also wondering how JLC customer service will handle this.