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

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

Query 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'

Query 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 for both queries are the same:

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'
mariadb mysql
1 Answers
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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)
user_61524000
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