What configs influence whether user credentials/grants are written to the binlog in Mysql
You are replicating only my_db.

There are valid security reasons for having the option to not replicate mysql.

The quick solution is to add binlog_do_db=mysql and restart.  However, any GRANTs and CREATE USERs have already been lost.  Either re-issue them and hope they replicate, or replay them on the Slave(s).