Parameter CLIENT_IDETIFIER is NULL whereas we got a literal value a few days ago

We used the parameter CLIENT_IDENTIFIER in our applications logic for logging and security issues. This parameter is described in

But suddenly, it has stopped working and next query returns NULL value:

SELECT SYS_CONTEXT ('USERENV', 'CLIENT_IDENTIFIER') FROM DUAL;

It doesn't matter if it's executed in SQL Developer, sqlplus, java application or whatever, always return NULL now.

Do you know the explanation for this change of behaviour and how can we go back to previous situation?

NOTE:

In the meanwhile, we have found a workaround using this other query:

select substr(program, 1, 30) from v$session where audsid = SYS_CONTEXT('USERENV','sessionid');