I agree, it doesn't make sense. I cannot recreate this either, it's working as expected for me. Did you rotate in edit mode or object mode and applied transformations? Which version of Blender are you using? I guess there are no modifiers on the object?! Would be best if you could share an example file.
This should be entirely straightforward as Prime8 says. Tangent normal direction is relative to UV orientation, not geometry - as explained in detail by FrankPolygon above, and as experienced by the entire game art community for at least the last 15 years. My guess would be that some sort of wanky transform on the mesh is…