@orobouros I think this is not that simple. In fact, this is a server-side script, which parses clear text message and HTMLifes it. When that server-side script encounters @ webfingerId of a remote user, it transforms it to a link correctly (just like in this message your webfingerId) or ignores it..., but when it encounters @ webfingerId of a local user, it dumbly converts to a link local part only, which produces an ugly HTML presentation. I would say that this is a server-side deficiency that is a consequence of a more general GNU Social deficiency in working with User GUIDs.
Please note that the same message may go to different instances, where @ localname references are not that obvious and may mislead reader...
@mmn