Wrong result with aggregation function, used inside window function

CREATE TABLE t (a INT);

INSERT INTO t VALUES(1),(2),(3),(4),(4);

SELECT a, SUM(a), SUM(a) OVER (ORDER BY SUM(a)) FROM t GROUP BY a;

SELECT a, SUM(a), SUM(a) OVER (ORDER BY SUM(a)+1) FROM t GROUP BY a; #  expected result 1,3,6,10

SELECT SUM(a) OVER (PARTITION BY SUM(a)) FROM t GROUP BY a;

SELECT SUM(a) OVER (PARTITION BY SUM(a)+1) FROM t GROUP BY a; #  expected result 1,3,6,10

MariaDB [test]> SELECT a, SUM(a), SUM(a) OVER (ORDER BY SUM(a)) FROM t GROUP BY a;

+------+--------+-------------------------------+

| a    | SUM(a) | SUM(a) OVER (ORDER BY SUM(a)) |

+------+--------+-------------------------------+

|    1 | 1      | 1                             |

|    2 | 2      | 3                             |

|    3 | 3      | 6                             |

|    4 | 8      | 10                            |

+------+--------+-------------------------------+

4 rows in set (0.00 sec)

MariaDB [test]> SELECT a, SUM(a), SUM(a) OVER (ORDER BY SUM(a)+1) FROM t GROUP BY a;

+------+--------+---------------------------------+

| a    | SUM(a) | SUM(a) OVER (ORDER BY SUM(a)+1) |

+------+--------+---------------------------------+

|    1 | 1      | 10                              |

|    2 | 2      | 10                              |

|    3 | 3      | 10                              |

|    4 | 8      | 10                              |

+------+--------+---------------------------------+

4 rows in set (0.01 sec)

MariaDB [test]> SELECT SUM(a) OVER (PARTITION BY SUM(a)) FROM t GROUP BY a;

+-----------------------------------+

| SUM(a) OVER (PARTITION BY SUM(a)) |

+-----------------------------------+

| 1                                 |

| 2                                 |

| 3                                 |

| 4                                 |

+-----------------------------------+

4 rows in set (0.00 sec)

MariaDB [test]> SELECT SUM(a) OVER (PARTITION BY SUM(a)+1) FROM t GROUP BY a;

+-------------------------------------+

| SUM(a) OVER (PARTITION BY SUM(a)+1) |

+-------------------------------------+

| 10                                  |

| 10                                  |

| 10                                  |

| 10                                  |

+-------------------------------------+

4 rows in set (0.00 sec)