This call should resolve to Germany, Hamburg, but the result is USA, City of Washington. 20038 is a valid zip code for Hamburg. Tests with other zip codes are ok. The rest service call should only resolve to german zip codes.
It is best to append the country to the end of your query if you want results from Germany to be returned. That said, for this German zip code I'm finding that it is not in the Bing Maps dataset. In fact it isn't in the dataset of any other online mapping
provider either. Based on this I would suspect that this is either a new zip code which hasn't yet been added to online maps or this zip code does not exist.