float3 computeNormalWithTBN(float3 vertexPosition, float2 texCoord, float3 normal ) {
    float3 p_dx = ddx(vertexPosition);
    float3 p_dy = ddy(vertexPosition);


    float2 tc_dx = ddx(texCoord);
    float2 tc_dy = ddy(texCoord);


    float3 t = normalize( tc_dy.y * p_dx - tc_dx.y * p_dy );
    float3 b = normalize( tc_dy.x * p_dx - tc_dx.x * p_dy );


    float3 n = normalize(normal);
    float3 x = cross(n, t);
    t = cross(x, n);
    t = normalize(t);


    x = cross(b, n);
    b = cross(n, x);
    b = normalize(b);


    float4 detail = normalMap.Sample( SampleType, texCoord );
    detail = (detail * 2.0f) - 1.0f;
    detail *= 6.0f;
    return normalize( normal + detail.x * t + detail.y * b );
}

// in main function
    // ...
    input.normal = computeNormalWithTBN( input.position.xyz, input.tex.xy, input.normal;
    float light = saturate( dot( input.normal, float3( 0, 0.73, -0.69 ) ) );


    float4 color = 0.3f;
    color += light;
    return color;
}

Why this happens?

XMFLOAT3 normal;
XMVECTOR v1 = {  2, height_value_of_3 - height_value_of_7, -2 };
XMVECTOR v2 = { -2, height_value_of_1 - height_value_of_9, -2 };
XMStoreFloat3( &normal, XMVector3Normalize( XMVector3Cross( v1, v2 ) ) );
return normal;