It only appears on the Arcaea realm map, not the island-wide dynamic map. Here's a close-up:
The realm maps are generated by fusing all the individual region polygons into a single realm polygon. When these two regions (Orbeh and Mraulaxon) are fused together, the divergence if the common border leaves a sliver-shaped hole. The map then has to outline that hole with a thick border. That causes the artifact you see. To fix it, the paths of one of the two regions would have to be redrawn to match the other.
