It’s because the texture and the UVs I’m assuming. The UVs are set up so that the same normal is sampled for each vertex loop. Where in the case of hard edge shading each vertex in a loop would need a separate normal. So the easiest way to go about fixing the issue would be cut the geometry were you wanted a hard edge. It would increase your vertex count and therefore the size of the texture, but it’s the first thing that comes to mind. Although you wouldn’t be able to have complex shading this way. Like a smooth round corner into a flat face you see in a lot of hard surface models. Well you could using the data transfer modifier in blender maybe not sure. I believe there is a modifier that automatically cuts your mesh where there are creases that would do the trick as well.