I don't see this as a bug. I also don't think it is a problem.
There is no difference in using
LongUserName inside of a comment when considering the amount of characters (aside from being off by 1). The only difference when using the @ is that it causes a "ping" or "notification".
Creating a comment which is little more than a ping to another user and a white space will be removed with flags. Users should know better than to do this because it is slightly rude to the user being notified.
If a user is doing this, flag their comment as "no longer needed". If they continue to do it, a moderator will probably issue a warning to them. If another user is doing this repeatedly to you, then flag their comment as "other" and type in a message explaining the situation.