Zabbix agent status on Zabbix server has some problems at the moment.

• Zabbix server make host unreachable if one item cannot retrieve data (network unreachable) from agent. if any user parameter item take a long time, Zabbix server stop monitoring all of other item on same host.

• If active agent is crashed, Zabbix server doesn't notice that.

• Agent status on host screen is not actually agent status (Zabbix server calculate internally)

I think it's good that Zabbix server and agent communicate using exclusive heartbeat connection periodically.
If the heartbeat is not successful, Zabbix server move host status to unreachable, and make a event.