zabbix_agentd didn't handle server requests correctly while multiple DNS records are used.
It does mean that:
- we've set the DNS record with multiple As (zabbix-servers.company.tld A 126.96.36.199, 188.8.131.52)
- we've set the reverse DNS records for both of this IP addresses to same domain (184.108.40.206 PTR zabbix-servers.company.tld; 220.127.116.11 PTR zabbix-servers.company.tld)
- we've set the zabbix_agentd Server option to this hostname (zabbix-servers.company.tld)
As result, zabbix_agentd refuse several servers requests (approximately 50%, I suppose that low-level getaddr round-robin is used).