Modify Naming attributes with custom advances format

I have tried modifying a username through the "Modify naming attributes" screen using a custom format we have developed (firstname.surname unless it's longer than 20 characters in which case initial.surname)

If i create a new user from a template using this format it works correctly but under the modify screen it just uses firstname.surname even if the name will be too long.

I've also noticed that when creating a user if the name is still too long it will show an error but through the modify screen it will quite happily update the user and truncate the windows 2000 logon name to 20 characters.

Any ideas on why the system is behaving this way?

Thanks