Problem w/ Migrate Source CSV importing Body content w/ HTML
You could try to use PHP's html_entity_decode() on the source field:

process:
  nid: nid
  title: title
  body/value: 
    plugin: callback
    callable: html_entity_decode
    source: body
  body/format:
    plugin: default_value
    default_value: full_html


PS: You've written that your source CSV doesn't contain encoded entities... have you checked the display settings for your field? (Manage &gt; Structure &gt; Content types &gt; NODE_TYPE &gt; Manage display). It could be that the import is correct, but the render settings are incorrectly set to something like plain text or a |raw filter in a Twig template file.