1. What happened
Skybox causes a strange shadow to fall onto a cube
2. How we can reproduce it using the example you attached
Note: There's only a cube, directional light, camera and the skybox in the scene.
To reproduce from scratch:
1. Create new project
2. Add a Cube
3. Stretch the cube out, so it's more like a plane
4. Scroll around looking at the cube

Alternatively:
1. Open attached project (further below Archive.zip)
2. Open Untitled scene
3. Camera is placed so that in game view shadow has artifacts
Expected result: The cube always looks smooth, with no shadows on it
Actual result: At some angles there's a visible shadow
GIF: https://gyazo.com/f65673b8e7e643bf4381f3ed15dcbedc