Many modifiers and other places use `CustomData_copy_data` to copy data
between different meshes. This function assumes that assumes that the
source and destination `CustomData` objects are "compatible" in some way.
Usually modifiers use `CustomData_copy` to create a compatible new
`CustomData` on the new mesh. The issue was that the optimization I added
for anonymous attributes broke this compatibility. It avoided copying some
attributes when they are no longer used.
This lead to attributes being copied incorrectly.
D13083 contains ideas for how this could be fixed more generally.
For now I just removed the optimization.
Differential Revision: https://developer.blender.org/D13083
ff4959eeaa