I suspect you are saving the file with color transformations... Try changing the Render settings in the Color Management panel to 'NonColour Data' or 'Linear Raw', and save the file again.
In my version of this node setup, if I plug the output of a NormalMap node (and a normal texture), into the node input, the difference between the normal texture and the node output is almost none.. Ideally it would be 0, but after three transforms, near 0 is good enough.
Those options are for different encodings of normal maps. In the game industry, tangent maps are very common because they are small. But in CG industry you have more formats around..Are you sure ? Then why is there a World Space option in the node ?
Anyway, the Normal Map node will not convert a world space vector into a tangent vector.