Wrong results from query, using expression with aggregated function and window function

create table t1(i int);

insert into t1 values  (1),(2),(3),(4),(5);

select sum(i) over (order by sum(i)) from t1 group by i;

select sum(i) over (order by sum(i)+1) from t1 group by i;

MariaDB [test]> select sum(i) over (order by sum(i)) from t1 group by i;

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

| sum(i) over (order by sum(i)) |

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

|                             1 |

|                             3 |

|                             6 |

|                            10 |

|                            15 |

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

5 rows in set (0.002 sec)

MariaDB [test]> select sum(i) over (order by sum(i)+1) from t1 group by i;

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

| sum(i) over (order by sum(i)+1) |

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

|                              15 |

|                              15 |

|                              15 |

|                              15 |

|                              15 |

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

5 rows in set (0.001 sec)

mysql> select sum(i) over (order by sum(i)) from t1 group by i;

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

| sum(i) over (order by sum(i)) |

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

|                             1 |

|                             3 |

|                             6 |

|                            10 |

|                            15 |

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

5 rows in set (0.00 sec)

mysql> select sum(i) over (order by sum(i)+1) from t1 group by i;

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

| sum(i) over (order by sum(i)+1) |

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

|                               1 |

|                               3 |

|                               6 |

|                              10 |

|                              15 |

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

5 rows in set (0.00 sec)