DDL COMPUTEONCHANGE works with CREATE TABLE but not with ALTER TABLE ADD COLUMN

I try to create a column with computeonchange. 

Its works with CREATE TABLE sql command, but if i use ALTER TABLE ADD COLUMN sql command this computeonchange doesn't works.

any reason why?

example:

CREATE TABLE MyStudents (   Name VARCHAR(16) NOT NULL,   surname VARCHAR(16))

alter table MyStudents add column fullname Varchar(50) COMPUTECODE { SET {fullname}={Name}_ "?" _{surname}} COMPUTEONCHANGE (Name,surname)  

insert into SQLUser.MyStudents (Name, surname) values ('name1',null)

update MyStudents set name = 'name' where name = 'name1'

this not updates fullname column value....

but it's works with a create table command.

CREATE TABLE MyStudents (   Name VARCHAR(16) NOT NULL,   surname VARCHAR(16),   fullname Varchar(50) COMPUTECODE { SET {fullname}={Name}_ "?" _{surname}} COMPUTEONCHANGE (Name,surname)  )