The difference in the work of two similar queries with the GROUP BY WITH ROLLUP clause in MariaDB

Question

Please help me understand why the following two queries return different results. The difference in the queries is only in the use of the DATE_FORMAT function in the GROUP BY clause.

Request 1:

SELECT DATE_FORMAT(T0.ET,'%Y/%m/%d %H%i'), SUM(T0.AT)
FROM(SELECT TIMESTAMPADD(second,(SEQ -1)*300, STR_TO_DATE('20170809135000','%Y%m%d%H%i%s')) ET, 1 AT
   FROM SEQ_1_TO_10
   WHERE SEQ <= 5) T0
   group by T0.ET with rollup 

Result 1:

'2017/08/09 1350', '1'
'2017/08/09 1355', '1'
'2017/08/09 1400', '1'
'2017/08/09 1405', '1'
'2017/08/09 1410', '1'
NULL, '5'

Request 2:

SELECT DATE_FORMAT(T0.ET,'%Y/%m/%d %H%i') , SUM(T0.AT)
FROM(SELECT TIMESTAMPADD(second,(SEQ -1)*300, STR_TO_DATE('20170809135000','%Y%m%d%H%i%s')) ET, 1 AT
   FROM SEQ_1_TO_10
   WHERE SEQ <= 5) T0
   group by  DATE_FORMAT(T0.ET,'%Y/%m/%d %H%i') with rollup 

Result 2:

'2017/08/09 1350', '1'
'2017/08/09 1355', '1'
'2017/08/09 1400', '1'
'2017/08/09 1405', '1'
'2017/08/09 1410', '1'
'2017/08/09 1410', '5'

EXPLAIN SELECT are the same for both queries:

id,  select_type, table,         type,    possible_keys, key,      key_len, ref,  rows, Extra
'1', 'SIMPLE',    'SEQ_1_TO_10', 'range', 'PRIMARY',     'PRIMARY','8',     NULL, '5',  'Using where; Using index; Using filesort'

Answer 1

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MariaDB [sandbox]> SELECT typ ,DATE_FORMAT(T.ET,'%Y/%m/%d %H%i') et, sum(1)
    -> FROM(
    -> SELECT TIMESTAMPADD(second,(SEQ -1)*300, STR_TO_DATE('20170809135000','%Y%m%d%H%i%s'))  ET,
    -> case when seq % 2 = 0 then 'a' else 'b' end as typ
    ->    FROM SEQ_1_TO_10
    ->    WHERE SEQ <= 5
    -> ) T
    -> group by typ,DATE_FORMAT(T.ET,'%Y/%m/%d %H%i') with rollup;
+------+-----------------+--------+
| typ  | et              | sum(1) |
+------+-----------------+--------+
| a    | 2017/08/09 1355 |      1 |
| a    | 2017/08/09 1405 |      1 |
| a    | 2017/08/09 1405 |      2 |
| b    | 2017/08/09 1350 |      1 |
| b    | 2017/08/09 1400 |      1 |
| b    | 2017/08/09 1410 |      1 |
| b    | 2017/08/09 1410 |      3 |
| NULL | 2017/08/09 1410 |      5 |
+------+-----------------+--------+
8 rows in set (0.00 sec)