Hello, welcome to ZBC.
An error like this could be a number of different things at play between the model, the normal map, and how whatever renderer you are using (of which there are so many) chooses to interpret this data.
Examples include how the model’s tangent basis is being calculated by the renderer, if it is making use of custom vertex normals on the model or just averaging them, and checking to ensure that the renderer is not incorrectly applying the wrong kind of compression or gamma settings to the image (most default to applying SRGB colorspace to texture maps instead of linear).
It’s worth noting that being able to see a seam on the normal map in the first image is not always indicative of any problem with the map itself. Normal maps contain data for computers to work with, not human eyes. So it’s entirely possible that the map is baked perfectly correct, and the problem with the shading stems more from the model or material settings being used by your renderer.
As a personal aside, when it comes to game engines I would recommend baking normal maps outside of zbrush. Programs that are more dedicated towards baking maps will give you a greater control over the settings, ensuring that the data you’re baking with will sync up specifically to the engine that will ultimately be rendering them.
