Currently API calls to GET search/tweets seem to be incorrectly flagging tweets with the key ‘favorited’ as False, even if the user has indeed favorited this status.
JSON response when attempting to test-create a favorite which is flagged incorrectly (with status code 403):
{“errors”:[{“code”:139,“message”:“You have already favorited this status”}]}
Manually browsing to the status using the web interface confirms that the tweet has been favorited already. AFAIK this behaviour seems to have begun as of today.