@pfaffman right, in the example you bring up it doesn’t make sense. However there are also many examples where a space is in order. Like at the beginning of the sentence as I just did, or saying »I like what @pfaffman said there«.
(Also, strangely enough a space is actually inserted after a mention. But the cursor is set before it. Do you know what I mean? This seems to point to that it’s actually intended that a space is inserted but there might be a bug @codinghorror?)