In two different browsers - if I try to enter @ArvoJ, then I type @Arv - editor displays me combo with ArvoJ - I press Enter or Tab key, @oJ is what results. Selecting user with mouse works.
Related bug (this is not exactly same problem, but I think they can be reported together) - if I type/edit @ArvoJ, then I type @Arv - then preview/post displays "@ArvoJ, then I type , then I type @Arv" - as you can see here.