JSON:API to Migrating Taxonomy References isn't mapping OK, for a number of reasons I'm doing a Drupal 8 - Drupal 8 migration using the JSON:API module on the source site.  I've run into an issue attaching taxonomy terms to nodes.  Here's as far as I've gotten : 

1) I created a vocabulary called "Photo Categories"

2) I've created and run import_drupal_taxonomy_photo_categories to import the terms.  This works perfectly.  I have manually checked to ensure they all get in.  I use the UUID as the migration's unique identifier.

3) I have a content type called Photos, it has a field called Photo Category which references the Photo Categories vocabulary.

4) From the source JSON in the relationships section I see this:

"field_photo_category": {
   "data": [
     {
       "type": "taxonomy_term--photo_categories",
       "id": "84c1e8bd-16f7-429f-b299-50fe43297d47"
     },
     {
       "type": "taxonomy_term--photo_categories",
       "id": "00931a31-d129-4c55-a962-f7ba11fed5a4"
     }
  ],
  "links": {
    "self": "https:\/\/seanreiser.com\/jsonapi\/node\/photo\/1396b571-0ece-4a07-ab17-7327e030dfb5\/relationships\/field_photo_category",
    "related": "https:\/\/seanreiser.com\/jsonapi\/node\/photo\/1396b571-0ece-4a07-ab17-7327e030dfb5\/field_photo_category"
  }
}


5) In the source > Fields section of import_drupal_type_photo I have:

- 
  name: field_photo_category
  label: 'Field Photo Category'
  selector: /relationships/field_photo_category/data


6) In the process section I have:

  field_photo_category:
    plugin: iterator
    source: field_photo_category
    process:
       plugin: migration_lookup
       migration: import_drupal_taxonomy_photo_categories
       source: id


As I understand it this should loop through the field_photo_category.data and do a migration lookup on each id.  But nothing is getting mapped.

I have verified that the ids match the ids in the data section match the UUID from the taxonomy term JSON.  I have checked the database and verfied that ids appear in migrate_map_import_drupal_taxonomy_photo_categories.soure1 and they map to the appropriate tids.

I have installed the migrate devel module and when I run the migration with --migrate-debug this is what the destination section look like:

  'field_photo_category' =&gt; array (2) [
        array (1) [
            'source' =&gt; string (36) "84c1e8bd-16f7-429f-b299-50fe43297d47"
        ]
        array (1) [
            'source' =&gt; string (36) "00931a31-d129-4c55-a962-f7ba11fed5a4"
        ]
    ]


So, my question, what am I doing wrong?  I have the rest of this part of my migration laid out and this seems to be the final obstacle.

Thanks in Advance
Entity reference fields need to fill their target_id column

field_photo_category:
  plugin: sub_process
  source: field_photo_category
  process:
    target_id:
       plugin: migration_lookup
       migration: import_drupal_taxonomy_photo_categories
       source: id