Cannot set subscriptions_status on migration from csv

Modules used: Simplenews and Migrate.

I have a single newsletter, to which I want to subscribe all users imported from the csv file.

This is my csv file content:

id,email,date_submitted

This is my yml import file

langcode: en
status: true
dependencies: {  }
id: test_subscriber
class: null
field_plugin_method: null
cck_plugin_method: null
migration_tags:
   - CSV
migration_group: test
label: 'Test Subscriber'
source:
  plugin: csv
  path: sites/default/import/subscriber.csv
  delimiter: ','
  enclosure: '"'
  header_row_count: 1
  track_changes: true
  ids:
    - id
process:
  type:
    plugin: default_value
    default_value: simplenews_subscriber
  created: date_submitted
  mail: email
  status:
    plugin: default_value
    default_value: 1
  subscriptions:
    plugin: sub_process
    source: subscriptions
    process:
      target_id:
        plugin: default_value
        default_value: my_newsletter
      status:
        plugin: default_value
        default_value: 1
      timestamp: date_submitted
      source:
        plugin: default_value
        default_value: website
destination:
  plugin: 'entity:simplenews_subscriber'
migration_dependencies:
  required: {  }
  optional: {  }

I can successfully import all the user with email and timestamp but the users are not subscribed to the newsletter (my_newsletter).

In database I have the table "simplenews_subscriber__subscriptions but the columns: "entity_id", "subscription_target_id", "subscription_status", "subscription_timestamp" and "subscriptions_source" doesn't get populated.

If I change the subscription setting in the yml file as follow:

    subscriptions:
      target_id:
        plugin: default_value
        default_value: my_newsletter
      status:
        plugin: default_value
        default_value: 1
      timestamp:
        plugin: default_value
        default_value: date_submitted
      source:
        plugin: default_value
        default_value: website

In database table "simplenews_subscriber__subscriptions, the columns: "entity_id" and "subscription_target_id" get populated but the columns: "subscription_status", "subscription_timestamp" and "subscriptions_source" are filled with null instead of data.

The end result is a list of user unsubscribed to my_newsletter (from subscriber edit page the my_newsletter checkbox is unchecked)

Thanks in advance to anybody willing to give me a tip.

I finally was able to solve the issue. Instead of this yml section code

subscriptions:
  target_id:
    plugin: default_value
    default_value: my_newsletter

I changed to this code

subscriptions/target_id:
  plugin: default_value
  default_value: my_newsletter

Here the full code

langcode: en
status: true
dependencies: {  }
id: test_subscriber
class: null
field_plugin_method: null
cck_plugin_method: null
migration_tags:
  - CSV
migration_group: test
label: 'Test Subscriber'
source:
  plugin: csv
  path: sites/default/import/subscriber.csv
  delimiter: ','
  enclosure: '"'
  header_row_count: 1
  track_changes: true
  ids:
    - id
process:
  type:
    plugin: default_value
    default_value: simplenews_subscriber
  created: date_submitted
  mail: email
  status:
    plugin: default_value
    default_value: 1
  subscriptions/target_id:
    plugin: default_value
    default_value: my_newsletter
  subscriptions/status:
    plugin: default_value
    default_value: 1
  subscriptions/timestamp:
    plugin: default_value
    default_value: date_submitted
  subscriptions/source:
    plugin: default_value
    default_value: website
destination:
  plugin: 'entity:simplenews_subscriber'
migration_dependencies:
  required: {  }
  optional: {  }