When the edit mesh modifier stack first deforms the original edit mesh,
it creates deformed positions in a separate array. That array is copied
to the mesh's vertex positions if a subsequent modifier requires an
actual mesh rather than a mesh wrapper.
However, if that last deform modifier is the last "on cage" modifier
and the next modifier requires a real Mesh, we hit a code path that
didn't copy the temporary position array, since it was contained in the
separate `EditMeshData` struct that must be handled manually.
This was a regression in 91b27ab637. In the future I hope to
make this simpler by expanding the use of implicit sharing and making
the conversion from original BMesh to a Mesh more lazy / const correct.
Hans Goudey