Exposed by383a145a19. Think the issue actually started incfa53e0fbe, since then the new destination normals were only filled from the first array copy onwards (probably hidden by some recalculation of normals somewhere?). In any case, starting the destination normals array with what we have from the input mesh normals and then resizing the array (instead of reinitializing it from scratch) fixes this. Pull Request: https://projects.blender.org/blender/blender/pulls/115612