poller_debug.txtSteps to reproduce:

1. Start Zabbix server and Zabbix agent
2. Have just a single host monitored with a single interface
3. Stop Zabbix agent
4. Read Zabbix server log

Result:

504929:20210826:153707.672 Zabbix agent item "agent.ping" on host "Zabbix server" failed: first network error, wait for 15 seconds
504929:20210826:153708.673 Zabbix agent item "system.cpu.util[,steal]" on host "Zabbix server" failed: first network error, wait for 15 seconds
504930:20210826:153723.698 Zabbix agent item "system.cpu.util[,interrupt]" on host "Zabbix server" failed: another network error, wait for 15 seconds 

There are randomly 1 to 5 seconds between "first network errors"
Sometimes only one "first network error" is reported

After that, 15 second interval is observed as expected

Debug log with debug level 4 for poller (504929)  and availability manager (504947) attached

Only one poller started, but the same observed with multiple pollers

Fresh Zabbix install

zabbix_server (Zabbix) 6.0.0alpha1
Revision f56fed83bc 25 August 2021, compilation time: Aug 25 2021 11:50:48 

Expected:
After "first network error" 15 seconds pause, then "another network error"