After creating/editing a ContentType with an XmlText fieldType in legacy, updating the fieldtype definition through the API fails with the following exception message:

  [eZ\Publish\Core\Base\Exceptions\ContentTypeFieldDefinitionValidationException]
  ContentType FieldDefinitions did not validate

The problem appears to be related to the tagPreset validation:

 object(eZ\Publish\Core\FieldType\ValidationError)#11617 (3) {
    ["singular":protected]=>
    string(44) "Setting '%setting%' is of unknown tag preset"
    ["plural":protected]=>
    NULL
    ["values":protected]=>
    array(1) {
      ["setting"]=>
      string(9) "tagPreset"
    }
  }

Steps to reproduce:

• Create a contentType (test) with an Xml field (body)
• Update the contentType's fieldType to change the description, for example (see linked gist)

          $fieldDefinition = $contentType->getFieldDefinition('body');
          $updateStruct = $contentTypeService->newFieldDefinitionUpdateStruct();
          $updateStruct->descriptions = array('eng-GB' => 'Body');
  

gist: https://gist.github.com/joaoinacio/de4492f763de8fd8441ef7e75aa8f218