Wrong result upon using FIRST_VALUE with a window frame

create table t1 (i int);

insert into t1 values (1),(2),(3),(4),(5),(6),(7),(8),(9),(10);

MariaDB [test]> select i, first_value(i) OVER ( order by i rows between current row and current row ) from t1;

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

| i    | first_value(i) OVER ( order by i rows between current row and current row ) |

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

|    1 |                                                                           1 |

|    2 |                                                                           1 |

|    3 |                                                                           1 |

|    4 |                                                                           1 |

|    5 |                                                                           1 |

|    6 |                                                                           1 |

|    7 |                                                                           1 |

|    8 |                                                                           1 |

|    9 |                                                                           1 |

|   10 |                                                                           1 |

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

10 rows in set (0.00 sec)

MariaDB [test]> select i, nth_value(i,1) OVER ( order by i rows between current row and current row ) from t1;

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

| i    | nth_value(i,1) OVER ( order by i rows between current row and current row ) |

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

|    1 |                                                                           1 |

|    2 |                                                                           2 |

|    3 |                                                                           3 |

|    4 |                                                                           4 |

|    5 |                                                                           5 |

|    6 |                                                                           6 |

|    7 |                                                                           7 |

|    8 |                                                                           8 |

|    9 |                                                                           9 |

|   10 |                                                                          10 |

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

10 rows in set (0.00 sec)

postgres=# select i, first_value(i) OVER ( order by i rows between current row and current row ) from t1;

 i  | first_value 

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

  1 |           1

  2 |           2

  3 |           3

  4 |           4

  5 |           5

  6 |           6

  7 |           7

  8 |           8

  9 |           9

 10 |          10

(10 rows)

postgres=# select i, nth_value(i,1) OVER ( order by i rows between current row and current row ) from t1;

 i  | nth_value 

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

  1 |         1

  2 |         2

  3 |         3

  4 |         4

  5 |         5

  6 |         6

  7 |         7

  8 |         8

  9 |         9

 10 |        10

(10 rows)