I did not have this issue when I first started participating here, but I am now running into the issue.
When I use the @reply system, I now have to finish typing out the full @username before clicking the name or the text in my post leaves off where I finished typing before clicking and it does not create an @reply link.
I am experiencing a similar issue with emoticons in the same way.
It seems that after you click "save edit" and sometimes for other reasons, the system will update and format it correctly. The issue seems to be that preview is having a tough time keeping up with some formatting related to @usernames and emoticons when you select them from the list without typing out the entire text (e.g., the full @username desired or the full emoticon such as : D or : smiley : without spaces)
If you were looking for a solution to this issue, try posting your content anyway and see if it formats it. Preview may not give an accurate preview.