Steps to reproduce:

1. Create a host with Agent interface
2. Create an item with "Zabbix agent (active)" type
3. Wait until some data appears in Monitoring -> Latest data
4. Observe host's availability in Configuration -> Hosts

Result:

Availability icon is gray which indicates that host's status is unknown (as per https://www.zabbix.com/documentation/current/manual/web_interface/frontend_sections/configuration/hosts). Though, documentation doesn't state that host status is unknown when only active items are being monitored.

Expected:

Either availability icon should be green (if data arrives), or documentation should state that host status is unknown when only active items are being monitored on the host.