The function parameter's collation returns NULL when the collation is defined in the function

10.4.31-dbg>SET @@local.character_set_connection='utf8mb4';

Query OK, 0 rows affected (0.000 sec)

10.4.31-dbg>

10.4.31-dbg>DELIMITER ||

10.4.31-dbg>CREATE FUNCTION f1(str TEXT CHARACTER SET utf8mb4 COLLATE utf8mb4_bin)

    -> RETURNS TEXT CHARACTER SET latin7 COLLATE latin7_bin

    -> BEGIN

    -> SET @param_coll = COLLATION(str);

    -> RETURN str;

    -> END ||

Query OK, 0 rows affected (0.016 sec)

10.4.31-dbg>DELIMITER ;

10.4.31-dbg>

10.4.31-dbg>SELECT COLLATION(f1('Hello, planet!'));

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

| COLLATION(f1('Hello, planet!')) |

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

| latin7_bin                      |

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

1 row in set (0.001 sec)

10.4.31-dbg>SELECT @param_coll;

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

| @param_coll |

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

| NULL        |

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

1 row in set (0.000 sec)

10.4.31-dbg>

10.4.31-dbg>SELECT @param_coll;

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

| @param_coll        |

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

| utf8mb4_bin        |

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

1 row in set (0.000 sec)

10.4.31-dbg>