If the report-host variable is not manually set on the slaves, the hostname or IP address is not automatically registered with the master and SHOW SLAVE HOSTS reports an empty string:
MariaDB [(none)]> show slave hosts;
This causes replication topology auto-discovery to be impractical, and advanced tools (such as failover management or replication hierarchy management tools) to be a pain to develop.
Suggestion is to set report-host with a default value, e.g. syscall to gethostname or gethostbyname.