as you may know this is the output of Alt + N >> Copy Vectors with multiple faces selected…
the goal is, having a given object with bad shading and an exact copy of it with good shading, to workaround copying normal data of each face in the object-copy and paste it on the equivalent face in the original object.
data transfer is made for this work but…
data transfer sometimes just acts up for me for some reason like shown.
data transfer is my go to to fix any shading issues.
I do encourage you to persist and figure out why it’s acting up on you.
any chance you can share the .blend file?
Mapping method “Projected Face Interpolated” seems to be the problem here.
I changed it to “Nearest Corner and Best Matching Face Normal” and it seems to fix the shading.