REVOKE ALL is not allowed for root

Server version:         10.5.2-MariaDB-log MariaDB Server

SQL> SELECT CURRENT_USER();

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

| CURRENT_USER() |

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

| root@localhost |

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

SQL> CREATE USER 'foo'@'bar';

SQL> REVOKE ALL PRIVILEGES ON *.* FROM 'foo'@'bar';

ERROR 1045 (28000): Access denied for user 'root'@'localhost' (using password: NO)

MariaDB 10.4.x

SQL> REVOKE ALL PRIVILEGES ON *.* FROM 'foo'@'bar';

Query OK, 0 rows affected, 1 warning (0.002 sec)