If it IS a bug, it's a usability bug. But to be fair to the Captivate developers, what else could they do when a user circumvents using the Insert Variable dialog other than just automatically assign the default number of characters (15 in your version of Captivate)? More recent versions of Cp now use 50 characters as the default.
All you have to do to avoid this 'bug' is to insert variables using the dialog method.