Doesn't bulk edit CCK fields with PHP

Note: while #358617: How to bulk edit cck fields? appears similar, it has evolved into something much different than this simple case of editing bulk CCK fields.

Steps to reproduce:

1. Add blank, new CCK text field Y to existing content type.
2. On view with VBO and only Modify node fields (views_bulk_operations_fields_action) selected, check the box next to Y.
3. In the Y's Code column, insert this:
   

   return array(
     0 => array('value' => $field_X[0]['view']),
   );

   (Field X is also a text field and has text in it.)

4. Select all nodes below.
5. Modify node fields
6. Confirm

When taken back to the view, I get this at the top:

3 rows processed in about 465 ms:

    * Performed Modify node fields on node A.
    * Performed Modify node fields on node B.
    * Performed Modify node fields on node C.

But when inspecting the nodes, field Y remains blank.