SHOW GRANTS does not prints a user's default role

MariaDB [(none)]> CREATE ROLE 'test_role';

Query OK, 0 rows affected (0.004 sec)

MariaDB [(none)]> CREATE USER 'test_user'@'%';

Query OK, 0 rows affected (0.004 sec)

MariaDB [(none)]> GRANT 'test_role' TO 'test_user'@'%';

Query OK, 0 rows affected (0.004 sec)

MariaDB [(none)]> SET DEFAULT ROLE 'test_role' FOR 'test_user'@'%';

Query OK, 0 rows affected (0.004 sec)

MariaDB [(none)]> SHOW CREATE USER 'test_user'@'%';

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

| CREATE USER for test_user@% |

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

| CREATE USER 'test_user'@'%' |

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

1 row in set (0.000 sec)

MariaDB [(none)]> SHOW GRANTS FOR 'test_user'@'%';

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

| Grants for test_user@%                |

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

| GRANT test_role TO 'test_user'@'%'    |

| GRANT USAGE ON *.* TO 'test_user'@'%' |

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

2 rows in set (0.000 sec)