Grants on 10.5+ server with pre-10.4 schema are silently ignored

If 10.5+ (i.e. a server with new privileges added in 10.5) is started on 10.3 or earlier datadir, and before mysql_upgrade is run, ACL statements are executed, then those which grant new privileges seemingly succeed but don't do anything. I suppose it has something to do with non-existing mysql.global_priv, and the old mysql.user not containing columns for the new privileges, but I would expect an error.

MariaDB [test]> create user u;

Query OK, 0 rows affected (0.001 sec)

MariaDB [test]> grant read_only admin on *.* to u;

Query OK, 0 rows affected (0.000 sec)

MariaDB [test]> show grants for u;

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

| Grants for u@%                |

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

| GRANT USAGE ON *.* TO `u`@`%` |

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

1 row in set (0.000 sec)