I've put over 2,000 miles on my Mariner D7 and no hints of handlepost issues so far. But....
Dahon stems are very flexy. If you're pulling back on the bars all the time (e.g. when climbing, or standing in the saddle) you are likely to have a failure like that. But....
If you've only put a few hundred miles on it, though, I'd assume that's a factory defect.