replicate_ignore_db='mysql' does not work on mariaDB 5.5.35

My purpose is to ignore replication for the mysql database.

I was trying to filter the replication for the mysql database in a master - slave configuration with mariaDB 5.5.35 , but I was not successful in the following configuration:

          Replicate_Do_DB: 

          Replicate_Ignore_DB: mysql

          Replicate_Do_Table: 

          Replicate_Ignore_Table: 

I tested this by creating a new user.

However with the following configuration it works:

         Slave_IO_Running: Yes

         Slave_SQL_Running: Yes

         Replicate_Do_DB: 

         Replicate_Ignore_DB: mysql,multitestrep

         Replicate_Do_Table: 

         Replicate_Ignore_Table: 

         Replicate_Wild_Do_Table: 

multitestrep is a test database created only for this purpose.
Unless I am not mistaking , it looks that the slave is ignoring the variable replicate_ignore_db if set global replicate_ignore_db='mysql'; but if the string contains another valid/existing database , then both databases are not replicated (this is what I need).

My purpose is to ignore replication for the mysql database and the only solution I found is to set replicate_ignore_db='mysql,multitestrep';

I tested this many times and it can be reproduced each time.

Regards.