HTML in Body field of a migration is not used, until the node is saved

Thanks for a fast reply. Below is an example. It is created by the WordPress Migrate module, and the same thing happens in #3535554: HTML tags in body text is shown as plain text:

uuid: d692021e-0022-450a-87f0-3ff3a244edec
langcode: en
status: true
dependencies: {  }
id: my_wordpress_content_post
class: null
field_plugin_method: null
cck_plugin_method: null
migration_tags:
  - WordPress
migration_group: my_wp
label: 'Import content from WordPress XML'
source:
  plugin: url
  data_fetcher_plugin: file
  data_parser_plugin: xml
  urls: {  }
  item_selector: '/rss/channel/item[wp:post_type="post"]'
  fields:
    -
      name: title
      label: 'Content title'
      selector: title
    -
      name: post_id
      label: 'Unique content ID'
      selector: 'wp:post_id'
    -
      name: content
      label: 'Body of the content'
      selector: 'content:encoded'
    -
      name: excerpt
      label: 'Excerpt of the content'
      selector: 'excerpt:encoded'
    -
      name: pubDate
      label: 'Publish date of the content'
      selector: pubDate
    -
      name: post_date
      label: 'Post date of the content'
      selector: 'wp:post_date'
    -
      name: status
      label: 'Published status of the content'
      selector: 'wp:status'
    -
      name: creator
      label: 'Username of the content creator'
      selector: 'dc:creator'
    -
      name: post_tag
      label: 'Tags assigned to the content item'
      selector: 'category[@domain="post_tag"]/@nicename'
    -
      name: category
      label: 'Categories assigned to the content item'
      selector: 'category[@domain="category"]/@nicename'
    -
      name: thumbnail_id
      label: 'Thumbnail assigned to the content item (Featured image)'
      selector: "wp:postmeta[wp:meta_key='_thumbnail_id']/wp:meta_value"
    -
      name: link
      label: 'Persistent path'
      selector: link
  ids:
    post_id:
      type: integer
process:
  title:
    -
      plugin: skip_on_empty
      method: row
      source: title
      message: 'Cannot import empty titles. This may include trashed posts from the WordPress import file.'
  _content/value:
    -
      plugin: get
      source: content
  _content/summary:
    -
      plugin: get
      source: excerpt
  created:
    -
      plugin: callback
      source: post_date
      callable: strtotime
  changed:
    -
      plugin: get
      source: '@created'
  status:
    -
      plugin: static_map
      source: status
      default_value: 0
      map:
        publish: 1
  path/pathauto:
    -
      plugin: default_value
      default_value: 0
  path/alias:
    -
      plugin: str_replace
      source: link
      search: 'http://ddev-wp-web'
      replace: ''
    -
      plugin: str_replace
      search: /\/$/
      replace: ''
      regex: true
    -
      plugin: str_replace
      search: '/^(?=^\/\?)(.*)$/'
      replace: ''
      regex: true
  uid:
    plugin: migration_lookup
    migration: my_wordpress_authors
    source: creator
  body: '@_content'
  _content/format:
    plugin: default_value
    default_value: full_html
  type:
    plugin: default_value
    default_value: article
  field_tags:
    plugin: migration_lookup
    migration: my_wordpress_categories
    source: category
  field_image:
    plugin: migration_lookup
    migration: my_wordpress_attachments
    source: thumbnail_id
destination:
  plugin: 'entity:node'
idMap: {  }
migration_dependencies:
  required:
    - my_wordpress_tags
    - my_wordpress_categories
    - my_wordpress_attachments
include: null

Your assumptions were correct @hongpong, and thanks for fixing this in the related WordPress Migrate module issue. Closing this issue, since the Drupal core Migrate module works well.