That is a really strange situation and one that I did not run into thankfully. My issue was that it did not map the featured images so I was left with 700 articles that I had to manually map images to. From what I saw when migrating from Drupal (7 in my case), the body content was pretty straight forward as the content area was probably left unaltered in the Drupal install I was migrating. My only suggestion would be that there may be a custom configuration at the Drupal end that is attached to the content body which is causing issues with the content mapping.
Just something that I wanted to add. Once you migrate the site, it will most likely tell you that you need to install Toolset Types to manage the Custom Post Types and the Taxonomies. Once you install Toolset Types, it pulls in Bootstrap as well which it says is needed to run the Toolset Types plugin and will not let you use it without it. Luckily I was able to map the CPT and Tax to the CPT UI plugin which is pretty cool. I replaced the Toolset Post, Term and Meta fields with ACF.
Not sure if this at all helped. Just giving some insight from what I went through with the FG plugin.
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