This was the case when the "Show in Editmode" option was used and a
vertexgroup affected the areas.
Probably an oversight in {rBdeaff945d0b9}?, seems like deforming
modifiers always need to call `BKE_mesh_wrapper_ensure_mdata` in
`deformVertsEM` when a vertex group is used.
Maniphest Tasks: T100578
Differential Revision: https://developer.blender.org/D15756
d269ad3535