How does "Flip Normals" affect Face Corner Data? 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:


Flipped Normals:
I would assume this is due to the fact that it is a bunch of disconnected planes which are treated like one object. In the newer Blender 2.8x+ series, normals are automatically corrected while editing in edit mode.
The problem in your case is that all of the disconnected planes are being treated as if they are one object, and therefore, the normals are going to look very messed up. However, if you try this with all the planes connected into a full, closed mesh, you should see this problem disappear when you recalculate (if needed) the normals. If you flip the normals, they should flip.