I believe that is the 60th parallel. It is the shadow of the 3D terrain over the 2D one.
This may or may not be an issue. Either way, you may find some minor inconsistencies throughout the world (as you can imagine it’s a big world) but nothing to worry about! :)
This is because you are at latitude 60 where we don’t have any elevation information above that and terrain is simply flatten to 0m altitude.
We plan on adding elevation data for terrain above 60 latitude in the future.