Understanding DB2 TIMESTAMP_FORMAT Format

I'd like to do a SELECT in DB2 in the format YYYY-MM-DD HH:MM:SS

on DB Column A: 20200605
Column B: 120000
Since it is defined as NUMBER type as shown in , the two columns were converted to CHAR type once, and after attaching them with CONCAT, TIME_STAMP_FORMAT converted them to date type in the format 'YYYY-MM-DD HH:24:MI:SS'.
Then, for some reason, the result was '2020-06-05-12.00.00.000000'.
I expected it to be 2020-06-05 12:00:00.

Try
SELECT TIMESTAMP_FORMAT(CONCAT(CHAR(20200101), CHAR(00000000))), 'YYYY-MM-DD HH:24:MI:SS')
FROM TEST_TABLE;
When I tested it as shown in , the result was '2020-01-01-00.00.00.000000'.
Is this the specification of TIMESTAMP_FORMAT?
If so, how can I get the results I'm looking for?