Oracle 11g Fast Add Column Taking 11 Hours

To the best of my understanding, a new feature of Oracle 11g is the ability to add a not null column with a default value very quickly as it would no longer auto-update every row in the table with the default value in the new column. However, the following command takes ~11 hours on a table with ~1.2 billion rows on an 11.2.0.4 database that had just been upgraded from 10.2.0.5:

alter table table_name add column_name varchar2(6) default "DEFLT' not null;

Although I've seen the following in the documentation:

"However, the optimized behavior is subject to the following restrictions:
•The table cannot have any LOB columns. It cannot be index-organized, temporary, or part of a cluster. It also cannot be a queue table, an object table, or the container table of a materialized view.
•The column being added cannot be encrypted, and cannot be an object column, nested table column, or a LOB column."

to the best of my knowledge my table does not meet any of these conditions. Describe shows no LOBs, ORGANIZATION INDEX isn't in the create statement, it isn't a temporary table, isn't a cluster, queue, object, or container table. And the column of course is a varchar column. Is there something I'm missing, or is the answer just that I'm mistaken about my table meeting one of these requirements?

ETA: Not sure if this is helpful, but I noticed in a paper that the tell-tell sign of fast column adds is the use of NVL in the filter predicate. From an explain plan on a test table, it does not appear that my database is executing fast adds:

SQL_ID  f78gwf6cz50uq, child number 0
-------------------------------------
select count(1) from t where z = 123456

Plan hash value: 1842905362

---------------------------------------------------------------------------
| Id  | Operation          | Name | Rows  | Bytes | Cost (%CPU)| Time     |
---------------------------------------------------------------------------
|   0 | SELECT STATEMENT   |      |       |       |   133 (100)|          |
|   1 |  SORT AGGREGATE    |      |     1 |    13 |            |          |
|*  2 |   TABLE ACCESS FULL| T    |   110K|  1406K|   133   (4)| 00:00:02 |
---------------------------------------------------------------------------

Predicate Information (identified by operation id):
---------------------------------------------------

   2 - filter("Z"=123456)

Note
-----
   - dynamic sampling used for this statement (level=2)