Migrate + JSON:API node references to entity type references aren't mapping

I have successfully migrated nodes from a D9 source site into a a custom entity type (RedHen CRM contact) on a target D9 site. Next, i am trying to migrate taxonomy terms with a multiple reference field from the source site into nodes of type 'networksn21' on the target site using the first migration. I am using the following yml file for the second migration:

source:
  plugin: url
  data_fetcher_plugin: http
  data_parser_plugin: json
  headers:
    Accept: 'application/json; charset=utf-8'
    Content-Type: 'application/json'
  urls: 
    - https://www.source-site.ch/jsonapi/taxonomy_term/netzwerk

  item_selector: data/
  fields:
    # ...other source fields...
    -
      # the multiple reference field on the source site
      # inside the taxonomy term 
      name: coordinators
      label: 'Coordinators'
      selector: /relationships/field_coordination/data

process:
  # ... other target fields ...
  # target field, multiple references to entities of type redhen_contact
  field_nwsn21_coord_contacts:
  plugin: sub_process
  source: coordinators
    process: 
      target_id:   
        plugin: migration_lookup
        # the previous migration into RedHen contacts:
        migration: crm_mig_contacts_sn21
        source: id # substituting 'target_id' here does not change outcome
   
migration_dependencies:
  required:
  - crm_mig_contacts_sn21

destination:
  plugin: 'entity:node'
  default_bundle: networksn21

The migration runs well for all simple fields but the references for the field_nwsn21_coord_contacts are not mapped at all (the target field remains empty)

If i run drush migrate:import crm_mig_networks_sn21 --migrate-debug, it displays the source array of references:

  "coordinators" => array:2 [
    0 => array:2 [
      "type" => "node--kontakt"
      "id" => "f5c235f1-fc0c-4755-bedf-59a5e294538f"
    ]
    1 => array:2 [
      "type" => "node--kontakt"
      "id" => "eec9b19d-4162-44ad-8810-f1dfbbff3beb"
    ]
  ]

In the 'destination' debug output, it see:

  "field_nwsn21_coord_contacts" => array:2 [
    0 => array:1 [
      "source" => "f5c235f1-fc0c-4755-bedf-59a5e294538f"
    ]
    1 => array:1 [
      "source" => "eec9b19d-4162-44ad-8810-f1dfbbff3beb"
    ]
  ]

The source keys and target ids are those i find in the migration mapping table and they are correct.

I do not have a real understanding of drupal migrate, just following some examples and the bits of documentation i could find on drupal.org. What do i do wrong? Any clues?