Odd behavior of character_set variables set to utf16 (when allowed)

MariaDB [test]> select @@character_set_connection;

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

| @@character_set_connection |

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

| utf8                       |

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

1 row in set (0.003 sec)

MariaDB [test]> set tx_isolation= 'READ-COMMITTED';

Query OK, 0 rows affected (0.000 sec)

MariaDB [test]> set character_set_connection=utf16;

Query OK, 0 rows affected (0.000 sec)

MariaDB [test]> set tx_isolation= 'READ-COMMITTED';

ERROR 1231 (42000): Variable 'tx_isolation' can't be set to the value of 'READ-COMMITTED'

MariaDB [test]> select @@character_set_filesystem, @@character_set_connection;

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

| @@character_set_filesystem | @@character_set_connection |

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

| binary                     | utf8                       |

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

1 row in set (0.000 sec)

MariaDB [test]> load data infile 'x' into table t1;

Query OK, 0 rows affected (0.000 sec)                

Records: 0  Deleted: 0  Skipped: 0  Warnings: 0

MariaDB [test]> set character_set_filesystem= utf16;

Query OK, 0 rows affected (0.000 sec)

MariaDB [test]> load data infile 'x' into table t1;

ERROR 1085 (HY000): The file '/data/bld/10.5-debug-nightly/data/test/' must be in the database directory or be readable by all

MariaDB [test]> set character_set_filesystem= utf8;

Query OK, 0 rows affected (0.000 sec)

MariaDB [test]> load data infile 'x' into table t1;

Query OK, 0 rows affected (0.000 sec)                

Records: 0  Deleted: 0  Skipped: 0  Warnings: 0