This type of black lines can appear on porcelain when using cutlery. The reason cannot be fully determined, but other manufacturers are experiencing the same thing.
However, there are some minerals in bright glaze (not to be confused with clear glaze) which tend to cause the surface to become slightly uneven over time. This unevenness is not visible to the naked eve, but “left-over” from the cutlery can leave black lines on the plate. This is also called deposits from the cutlery, which will appear as black lines.
It is recommended to use a ceramic eraser to remove these.