I use the "Normal Edit" and "Data Transfer" functions a lot, therefore I'm working with custom Normals with Face Corner Normals that differ from the Vertex Normals. The Pink lines represent the Face Corner Normals that are all pointing downwards towards the Cube:
After I Flip Normals this happens.
The two diagonal pairs of Face Corner Normals point into a different direction which results in odd definitely not intended shading:
I can fix the issue with the Data Transfre module and a bit of hassle. But I would like to know what the **** is going on here? I want to know how blender calculates the new position of Face Corner Data.
If your IQ is above 160 maybe you can figure it out because I can't, rotations are in precisely 15degree increments: