There is a code in PhysicallyBasedSkyRenderer
float r = Vector3.Distance(builtinParams.worldSpaceCameraPos, pbrSky.planetCenterPosition.value);
float R = pbrSky.planetaryRadius.value;
bool isPbrSkyActive = true;//r > R; // Disable sky rendering below the ground
here planetary radius and planet center is in km, but worldSpaceCameraPos in m, if I understand correctly, which will lead to incorrect results.