This was a regression in deaff945d0 which skips copying a mesh.
Dupli-verts/faces were not updated to account for this.
This supports iterating over edit-mesh vertices & faces,
since falling back to a full copy (as we do in some places)
will be slow while transforming geometry.
This commit looks as if it would change behavior with orcos,
since any edit-mesh deformation causes them to be assigned.
However in practice there is no functional change, details in comments.
ad05e1100f