It is not a bug. The name fields all need content. The default current last name is the last name at birth. If you remove that name, the field still requires an entry. If you do not input a name, the default will be "Anonymous".
A Current Last Name is not like the Last Name At Birth. Changing a CLN can be done quite easily, without causing the same kind of issues a LNAB change can cause.
The CLN can be changed any number of times, when new information is found. I have done this when I have found a later marriage, after already using the previous married name as the CLN.