resolve dns name

[scypi]

I have private subnet to monitor my hosts and on several of them want check if sshd is running on public interface.
For this purpose I use this template

net.service[sshd,{HOST.DNS},22]

and works on most of hosts but few of them reports service down.
Making some checks manually

zabbix# zabbix_get -s 172.16.10.1 -k "net.tcp.service[ssh,example1.com,22]"
0
zabbix# host example1.com
example1 has address 1.1.1.1
zabbix# zabbix_get -s 172.16.10.1 -k "net.tcp.service[ssh,1.1.1.1,22]"
1

this is a problem with resolve dns name, but ..

example1.com# host example1.com
example1 has address 1.1.1.1

I wanted to ask where and how they are resolved DNS name, such as Zabbix server and the monitored machine have done correctly.

Yet I found such a difference

zabbix# zabbix_get -s 172.16.10.1 -k "net.tcp.service[ssh,example1.com,22]"
0
zabbix# zabbix_get -s 172.16.10.1 -k "agent.version"
1.8.7
zabbix# zabbix_get -s 172.16.0.2 -k "net.tcp.service[ssh,example2.com,22]"
1
zabbix# zabbix_get -s 172.16.0.2 -k "agent.version"
1.8

Do newer versions have a problem with resolve dns name in checks?