why the column DBA_AUDIT_PRIVILEGE.SYS_PRIVILEGE is not populated?

Hello, since a time ago this column is empty in the dba_audit_trail. why is that? These are the difined audit: (Database Enterpsie Edition 11.2.0.4.)

USER_NAME PROXY_NAME AUDIT_OPTION SUCCESS FAILURE

GRANT ANY PRIVILEGE BY ACCESS BY ACCESS

SYSTEM GRANT BY ACCESS BY ACCESS

GRANT ANY ROLE BY ACCESS BY ACCESS

DIRECTORY BY ACCESS BY ACCESS

PROCEDURE BY ACCESS BY ACCESS

VIEW BY ACCESS BY ACCESS

USER BY ACCESS BY ACCESS

TABLE BY ACCESS BY ACCESS

CREATE SESSION NOT SET BY ACCESS

ALTER SYSTEM BY ACCESS BY ACCESS

OLRUEDAR ALTER SYSTEM BY ACCESS BY ACCESS

thanks,

Andrea Lozano