Drupal-to-Drupal data migration Taxonomy issues
Well, this is a couple years' old now, but thank you doodirock for tickling my memory on this. You need to configure that field's source_type value to 'tid'. I'm doing so in code, but I believe that you can set this in the UI. The code you need is this:

$this-&gt;addFieldMapping('field_news_categories:source_type')
  -&gt;defaultValue('tid');


In my UI view, this row appears like this:



Where 'tid' is the value input. 

As the documentation points out:


  source_type: If the option value is 'tid', then the incoming value is assumed to be a valid term ID in the destination Drupal instance. Otherwise, it is assumed to be a term name (the default behavior). Note that you'll want to set this to 'tid' if the mapping has a source migration (the sourceMigration() method is used on the mapping).