the value of undo_sql in flashback_transaction_query view is always null

Question

so I have a table defined as follow:

`create table test.test as (select * from dual);

and ran the following update statements:

update test.test set dummy = '1' where dummy = 'X';
commit;

I want to find the undo_sql for the transaction above, so I ran the following statement to find the transaction id

SQL> select dummy, versions_xid, versions_startscn, versions_endscn from test.test
versions between scn minvalue and maxvalue ;

D VERSIONS_XID     VERSIONS_STARTSCN VERSIONS_ENDSCN
- ---------------- ----------------- ---------------
1 0A000500B00A0000           2802291
X                                            2802291

and ran the following statement to find the undo_sql statement

SQL> select undo_sql from flashback_transaction_query 
where xid = (select  versions_xid from test.test versions 
between scn minvalue and maxvalue where versions_xid is not null);

UNDO_SQL
--------------------------------------------------------------------------------

as you can see, I got nothing, if I run the following statement, I also get nothing.

SQL> select undo_sql from flashback_transaction_query where undo_sql is not null;

no row selected

looks like undo_sql is always null;

Is there anything I didn't configure ? Here are the values from v$system_parameter

SQL> select name, value from v$system_parameter where name like '%undo%';

NAME                 VALUE
----------------     ----------------
undo_management      AUTO
undo_tablespace      UNDOTBS1
undo_retention       18000