Zabbix proxy sends availability data to the server. It is a JSON object like:

{
"data":[

{ "hostid":10101, "available":1, "snmp_available":0 }

,

{ "hostid":10104, "jmx_available":1 }

]
}

The server updates availability in configuration cache only for the last host in JSON object, i.e. for host 10104.

Related issue: ZBX-4991