Why doesn't skip_on_empty work when migrating from a text field?

Question

I'm trying to migrate Drupal 7 users to Drupal 8. In Drupal 7, there's an additional text field representing the user's group. In Drupal 8, it's a taxonomy vocabulary. So I'm using the following pipeline:

  field_user_group:
    -
      plugin: skip_on_empty
      method: process
      source: field_directorate
    -      
      plugin: extract
      index:
        - 0
        - value
    -
      plugin: entity_generate
      value_key: name
      bundle_key: vid
      bundle: groups
      entity_type: taxonomy_term
      ignore_case: true

field_directorate is the source field in D7, and field_user_group is the target in D8.

Records without that value set should be caught by the skip_on_empty plugin, and left blank, but that plugin seems to be failing (letting the process continue to the next plugin in the pipeline) as I'm getting errors from the extract plugin saying:

Array index missing, extraction failed.

According to the documentation,

The skip_on_empty process plugin checks to see if the current input value is empty (empty string, NULL, FALSE, 0, '0', or an empty array). If so, the further processing of the property or the entire row (depending on the chosen method) is skipped and will not be migrated.

How is the processing even getting to extract if skip_on_empty is supposed to skip values with an empty array?

It works fine for records with the value set.

Answer 1

I now believe this to be caused by Get process plugin sets multiple for empty array.