Incorrect estimation time (kqlfxpl_time)

My Story...

1) the db is upgrade from 11g to 12c by using DBUA

2) In execution plan, the column "Time" always show 00:00:01

3) I try to run a full table scan operation(4.6G), the estimation time is 7 second (the real elapsed should be around 20 min)

4) My DB set optimizer_adaptive_reporting_only=true

5) I have traced the source of V$SQL_PLAN, finally found that the value of "kqlfxpl_time" at "x$kqlfxpl" is 1 or 0.....Why.. ?

SQL> select * from pfd01.abcd_hist;

Execution Plan

----------------------------------------------------------

Plan hash value: 2410447193

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| Id  | Operation         | Name              | Rows  | Bytes | Cost (%CPU)| Time     |

---------------------------------------------------------------------------------------

|   0 | SELECT STATEMENT  |                   |    49M|  4624M|   171K  (1)| 00:00:07 |