The space is used as delimiter for the @-reference; this means that if you write "@Longname with spaces," the notification would be sent to a user with a username matching "Longname" that commented before you, including the user using "Longname with spaces" as username.
In your case, if I write "@Erik Dekker" in a comment, the notification would be for any user with a username matching "Erik"; supposing that Erik commented after you, I would notify him, not you. To avoid ambiguity, the autocomplete suggest "ErikDekker," which is how your username is transformed when finding the user to whom the message is directed.
If the space were not used as delimiter, when you would write a comment similar to the following one the system would try to notify a user whose username is the full message.
@kiamlaluno I have already tried as you suggested.